fix(anthropic): restrict fast mode to Opus 4.6 (Anthropic API contract)

Per https://platform.claude.com/docs/en/build-with-claude/fast-mode:
"Fast mode is currently supported on Opus 4.6 only. Sending speed: fast
with an unsupported model returns an error."

Pre-fix, _is_anthropic_fast_model() returned True for any claude-* model,
so /fast on Opus 4.7 (or Sonnet/Haiku) would persist agent.service_tier=fast
in config.yaml and the adapter would inject extra_body["speed"] = "fast"
on every subsequent request. Opus 4.7 returns:

  HTTP 400: 'claude-opus-4-7' does not support the `speed` parameter.

This wedged sessions across model upgrades (a user who ran /fast on Opus 4.6
and later switched the default model to 4.7 hit a hard 400 on every turn
until they manually edited config.yaml).

Changes:
- _is_anthropic_fast_model: gate on "opus-4-6" / "opus-4.6" only
- anthropic_adapter: add _supports_fast_mode predicate as defensive guard
  so stale request_overrides on an unsupported model are dropped silently
  instead of 400'ing
- Tests: flip the assertions that mirrored the bug (Sonnet/Haiku/Opus 4.7
  asserting fast-mode support) to match the documented API contract

tests/agent/test_anthropic_adapter.py

@@ -1113,6 +1113,45 @@ class TestBuildAnthropicKwargs:
         assert _forbids_sampling_params("claude-opus-4-6") is False
         assert _forbids_sampling_params("claude-sonnet-4-5") is False
 
+    def test_supports_fast_mode_predicate(self):
+        """Fast mode is Opus 4.6 only — Opus 4.7 and others must be excluded."""
+        from agent.anthropic_adapter import _supports_fast_mode
+        assert _supports_fast_mode("claude-opus-4-6") is True
+        assert _supports_fast_mode("anthropic/claude-opus-4-6") is True
+        assert _supports_fast_mode("claude-opus-4-7") is False
+        assert _supports_fast_mode("claude-sonnet-4-6") is False
+        assert _supports_fast_mode("claude-haiku-4-5") is False
+        assert _supports_fast_mode("") is False
+
+    def test_fast_mode_omitted_for_unsupported_model(self):
+        """fast_mode=True on Opus 4.7 must NOT inject speed=fast (API 400s)."""
+        kwargs = build_anthropic_kwargs(
+            model="claude-opus-4-7",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=None,
+            max_tokens=1024,
+            reasoning_config=None,
+            fast_mode=True,
+        )
+        # extra_body either absent or doesn't carry "speed"
+        assert "speed" not in kwargs.get("extra_body", {})
+        # No fast-mode beta header should be added either
+        beta_header = (kwargs.get("extra_headers") or {}).get("anthropic-beta", "")
+        assert "fast-mode-2026-02-01" not in beta_header
+
+    def test_fast_mode_still_applied_on_opus_46(self):
+        """Regression guard — fast mode must still work on Opus 4.6."""
+        kwargs = build_anthropic_kwargs(
+            model="claude-opus-4-6",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=None,
+            max_tokens=1024,
+            reasoning_config=None,
+            fast_mode=True,
+        )
+        assert kwargs.get("extra_body", {}).get("speed") == "fast"
+        assert "fast-mode-2026-02-01" in kwargs["extra_headers"]["anthropic-beta"]
+
     def test_reasoning_disabled(self):
         kwargs = build_anthropic_kwargs(
             model="claude-sonnet-4-20250514",