fix(llm): apply DeepSeek capabilities to OpenRouter-namespaced models

- OpenRouter exposes DeepSeek as deepseek/<id>, which matched neither the exact
  IDs nor the patterns, so a thinking model like deepseek/deepseek-v4-flash fell
  through to _DEFAULT and had object-form tool_choice forced on it
- strip the official deepseek/ namespace before lookup so it reuses the native
  quirks; deepseek/deepseek-chat still keeps tool_choice, and third-party
  finetunes on other publishers stay on _DEFAULT #1199

tests/test_capabilities.py

@@ -116,6 +116,37 @@ class TestDefault:
         assert caps.supports_tool_choice is True
 
 
+@pytest.mark.unit
+class TestOpenRouterDeepSeekNamespace:
+    """OpenRouter namespaces DeepSeek as ``deepseek/<id>``; strip it so the
+    same quirks apply as the native provider (#1199)."""
+
+    def test_prefixed_v4_flash_suppresses_tool_choice(self):
+        # Was falling through to _DEFAULT (tool_choice on) -> slow object-form call.
+        assert get_capabilities("deepseek/deepseek-v4-flash").supports_tool_choice is False
+
+    def test_prefixed_reasoner_suppresses_tool_choice(self):
+        assert get_capabilities("deepseek/deepseek-reasoner").supports_tool_choice is False
+
+    def test_prefixed_chat_selects_deepseek_chat_not_default(self):
+        # Must resolve to _DEEPSEEK_CHAT, not _DEFAULT: supports_json_schema=False
+        # is what distinguishes them (both keep tool_choice).
+        caps = get_capabilities("deepseek/deepseek-chat")
+        assert caps.supports_tool_choice is True
+        assert caps.supports_json_schema is False  # _DEEPSEEK_CHAT, not _DEFAULT
+
+    def test_only_official_namespace_is_stripped(self):
+        # A third-party publisher whose model name WOULD match a deepseek pattern
+        # must stay _DEFAULT: proves we strip only "deepseek/", not any "*/".
+        caps = get_capabilities("tngtech/deepseek-v4-flash")
+        assert caps.supports_tool_choice is True         # not thinking
+        assert caps.supports_json_schema is True          # _DEFAULT
+
+    def test_native_ids_unchanged(self):
+        assert get_capabilities("deepseek-v4-flash").supports_tool_choice is False
+        assert get_capabilities("deepseek-chat").supports_tool_choice is True
+
+
 @pytest.mark.unit
 def test_capabilities_dataclass_is_frozen():
     """Capability rows are immutable so they can be safely shared."""