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>

run_agent.py

@@ -9798,7 +9798,6 @@ class AIAgent:
         )
         _is_tokenhub = base_url_host_matches(self._base_url_lower, "tokenhub.tencentmaas.com")
         _is_lmstudio = (self.provider or "").strip().lower() == "lmstudio"
-        _is_deepseek = base_url_host_matches(self.base_url, "api.deepseek.com")
 
         # Temperature: _fixed_temperature_for_model may return OMIT_TEMPERATURE
         # sentinel (temperature omitted entirely), a numeric override, or None.
@@ -9910,7 +9909,6 @@ class AIAgent:
             is_kimi=_is_kimi,
             is_tokenhub=_is_tokenhub,
             is_lmstudio=_is_lmstudio,
-            is_deepseek=_is_deepseek,
             is_custom_provider=self.provider == "custom",
             ollama_num_ctx=self._ollama_num_ctx,
             provider_preferences=_prefs or None,
@@ -10370,11 +10368,6 @@ class AIAgent:
         # context compaction).  Don't pass null to the API.
         api_msg.pop("reasoning_content", None)
 
-        # DeepSeek: strip reasoning_content on all assistant messages so the API
-        # doesn't return 400 when the model was invoked with thinking enabled.
-        if base_url_host_matches(self.base_url, "api.deepseek.com"):
-            api_msg.pop("reasoning_content", None)
-
     @staticmethod
     def _sanitize_tool_calls_for_strict_api(api_msg: dict) -> dict:
         """Strip Codex Responses API fields from tool_calls for strict providers.