fix(deepseek): wire thinking-mode via DeepSeekProfile, not legacy fallback

The cherry-picked PR #15251 from @tw2818 correctly identified the
DeepSeek 400 root cause but placed the fix in the legacy fallback path
of `build_kwargs`, which DeepSeek never reaches — DeepSeek has a
registered ProviderProfile and goes through `_build_kwargs_from_profile`
instead. The legacy-path block was therefore dead code.

This commit pivots the fix to where it actually fires:

- New `DeepSeekProfile` in `plugins/model-providers/deepseek/__init__.py`
  overrides `build_api_kwargs_extras` to emit DeepSeek's expected wire
  format (mirrors `KimiProfile`):

      {"reasoning_effort": "<low|medium|high|max>",
       "extra_body": {"thinking": {"type": "enabled" | "disabled"}}}

- Model gating: only `deepseek-v4-*` and `deepseek-reasoner` emit
  thinking control. `deepseek-chat` (V3) is untouched — current behavior.

- Effort mapping: low/medium/high passthrough, xhigh/max → max, unset →
  omitted (DeepSeek server applies its own default).

- Revert the legacy-path additions from PR #15251 — they were dead code,
  and the `_copy_reasoning_content_for_api` strip block specifically
  would have nullified the existing reasoning_content padding machinery
  (`_needs_deepseek_tool_reasoning` → space-pad on replay) that the
  active provider already relies on for replay correctness.

- Unit tests pin the wire-shape contract and the model gating rules
  (26 tests, all passing). Existing transport + provider profile suites
  (321 tests) continue to pass.

- AUTHOR_MAP: map twebefy@gmail.com → tw2818 for release notes credit.

Closes #15700, #17212, #17825.
Co-authored-by: tw2818 <twebefy@gmail.com>

tests/plugins/model_providers/test_deepseek_profile.py

@@ -0,0 +1,184 @@
+"""Unit tests for the DeepSeek provider profile's thinking-mode wiring.
+
+DeepSeek V4 (and the legacy ``deepseek-reasoner``) expects every request to
+carry an explicit ``extra_body.thinking`` parameter.  Omitting it makes the
+server default to thinking-mode ON, which then enforces the
+``reasoning_content``-must-be-echoed-back contract on subsequent turns and
+breaks the conversation with HTTP 400 (#15700, #17212, #17825).
+
+These tests pin the profile's wire-shape contract so DeepSeek requests stay
+correctly shaped without going live.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+
+@pytest.fixture
+def deepseek_profile():
+    """Resolve the registered DeepSeek profile.
+
+    Going through ``providers.get_provider_profile`` keeps the test honest —
+    if someone later replaces the registered class with a plain
+    ``ProviderProfile``, every assertion below collapses.
+    """
+    # ``model_tools`` triggers plugin discovery on import, which is what
+    # registers the DeepSeek profile in the global provider registry.
+    import model_tools  # noqa: F401
+    import providers
+
+    profile = providers.get_provider_profile("deepseek")
+    assert profile is not None, "deepseek provider profile must be registered"
+    return profile
+
+
+class TestDeepSeekThinkingWireShape:
+    """``build_api_kwargs_extras`` produces DeepSeek's exact wire format."""
+
+    def test_v4_pro_default_enables_thinking_without_effort(self, deepseek_profile):
+        """No reasoning_config → thinking enabled, server picks default effort."""
+        extra_body, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config=None, model="deepseek-v4-pro"
+        )
+        assert extra_body == {"thinking": {"type": "enabled"}}
+        assert top_level == {}
+
+    def test_v4_pro_enabled_with_high_effort(self, deepseek_profile):
+        extra_body, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": True, "effort": "high"},
+            model="deepseek-v4-pro",
+        )
+        assert extra_body == {"thinking": {"type": "enabled"}}
+        assert top_level == {"reasoning_effort": "high"}
+
+    @pytest.mark.parametrize("effort", ["low", "medium", "high"])
+    def test_standard_efforts_pass_through(self, deepseek_profile, effort):
+        _, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": True, "effort": effort},
+            model="deepseek-v4-pro",
+        )
+        assert top_level == {"reasoning_effort": effort}
+
+    @pytest.mark.parametrize("effort", ["xhigh", "max", "MAX", "  Max  "])
+    def test_xhigh_and_max_normalize_to_max(self, deepseek_profile, effort):
+        _, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": True, "effort": effort},
+            model="deepseek-v4-pro",
+        )
+        assert top_level == {"reasoning_effort": "max"}
+
+    def test_explicitly_disabled_sends_disabled_marker(self, deepseek_profile):
+        """``reasoning_config.enabled=False`` → ``thinking.type=disabled``.
+
+        The crucial bit is that the parameter is *sent* at all — DeepSeek
+        defaults to thinking-on when ``thinking`` is absent.
+        """
+        extra_body, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": False}, model="deepseek-v4-pro"
+        )
+        assert extra_body == {"thinking": {"type": "disabled"}}
+        # No effort when disabled — DeepSeek rejects it.
+        assert top_level == {}
+
+    def test_disabled_ignores_effort_field(self, deepseek_profile):
+        """Effort silently dropped when thinking is off."""
+        _, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": False, "effort": "high"},
+            model="deepseek-v4-pro",
+        )
+        assert top_level == {}
+
+    def test_unknown_effort_omits_top_level(self, deepseek_profile):
+        """Garbage effort → omit reasoning_effort so DeepSeek applies its default."""
+        _, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": True, "effort": "garbage"},
+            model="deepseek-v4-pro",
+        )
+        assert top_level == {}
+
+    def test_empty_effort_omits_top_level(self, deepseek_profile):
+        _, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": True, "effort": ""},
+            model="deepseek-v4-pro",
+        )
+        assert top_level == {}
+
+
+class TestDeepSeekModelGating:
+    """V4 family + ``deepseek-reasoner`` get thinking; V3 stays untouched."""
+
+    @pytest.mark.parametrize(
+        "model",
+        [
+            "deepseek-v4-pro",
+            "deepseek-v4-flash",
+            "deepseek-v4-future-variant",
+            "deepseek-reasoner",
+            "DEEPSEEK-V4-PRO",  # case-insensitive
+        ],
+    )
+    def test_thinking_capable_models_emit_thinking(self, deepseek_profile, model):
+        extra_body, _ = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config=None, model=model
+        )
+        assert extra_body == {"thinking": {"type": "enabled"}}
+
+    @pytest.mark.parametrize(
+        "model",
+        [
+            "deepseek-chat",         # V3 alias
+            "deepseek-v3-0324",      # explicit V3
+            "deepseek-v3.1",         # V3 minor revisions
+            "",                       # bare/unknown
+            None,                     # missing
+            "deepseek-unknown",      # unrecognized
+        ],
+    )
+    def test_non_thinking_models_emit_nothing(self, deepseek_profile, model):
+        extra_body, top_level = deepseek_profile.build_api_kwargs_extras(
+            reasoning_config={"enabled": True, "effort": "high"}, model=model
+        )
+        assert extra_body == {}
+        assert top_level == {}
+
+
+class TestDeepSeekFullKwargsIntegration:
+    """End-to-end: the transport's full kwargs match DeepSeek's live wire format.
+
+    The live test harness in ``tests/run_agent/test_deepseek_v4_thinking_live.py``
+    sends ``{"reasoning_effort": "high", "extra_body": {"thinking": {"type":
+    "enabled"}}}``.  Confirm the transport produces that exact shape when wired
+    through the registered DeepSeek profile.
+    """
+
+    def test_full_kwargs_match_live_wire_shape(self, deepseek_profile):
+        from agent.transports.chat_completions import ChatCompletionsTransport
+
+        kwargs = ChatCompletionsTransport().build_kwargs(
+            model="deepseek-v4-pro",
+            messages=[{"role": "user", "content": "ping"}],
+            tools=None,
+            provider_profile=deepseek_profile,
+            reasoning_config={"enabled": True, "effort": "high"},
+            base_url="https://api.deepseek.com/v1",
+            provider_name="deepseek",
+        )
+        assert kwargs["model"] == "deepseek-v4-pro"
+        assert kwargs["reasoning_effort"] == "high"
+        assert kwargs["extra_body"] == {"thinking": {"type": "enabled"}}
+
+    def test_v3_chat_full_kwargs_omit_thinking(self, deepseek_profile):
+        from agent.transports.chat_completions import ChatCompletionsTransport
+
+        kwargs = ChatCompletionsTransport().build_kwargs(
+            model="deepseek-chat",
+            messages=[{"role": "user", "content": "ping"}],
+            tools=None,
+            provider_profile=deepseek_profile,
+            reasoning_config={"enabled": True, "effort": "high"},
+            base_url="https://api.deepseek.com/v1",
+            provider_name="deepseek",
+        )
+        assert "reasoning_effort" not in kwargs
+        assert "extra_body" not in kwargs or "thinking" not in kwargs.get("extra_body", {})