fix(llm): gate MiniMax reasoning_split by model capability (#826)

MinimaxChatOpenAI unconditionally set reasoning_split=True, but the
kwarg is only valid on M2.x reasoning models. The openai SDK's strict
kwarg validation raised TypeError for Coding Plan and any other non-
reasoning MiniMax model.

Adds requires_reasoning_split to ModelCapabilities, gates the payload
injection on it, and only sets True for _MINIMAX_THINKING (M2.x exact
IDs and the ^MiniMax-M\d forward-compat pattern). Same shape as the
existing supports_tool_choice gate.

Regression tests cover both halves: M2.x models still receive the flag,
non-reasoning MiniMax models do not.

tests/test_capabilities.py

@@ -75,6 +75,22 @@ class TestMinimaxExactMatches:
     def test_m2_base_rejects_tool_choice(self):
         assert get_capabilities("MiniMax-M2").supports_tool_choice is False
 
+    def test_m2_x_requires_reasoning_split(self):
+        # M2.x reasoning models need reasoning_split=True so <think> blocks
+        # land in reasoning_details instead of content (#826).
+        for model in ("MiniMax-M2.7", "MiniMax-M2.5-highspeed", "MiniMax-M2"):
+            assert get_capabilities(model).requires_reasoning_split is True
+
+    def test_future_m3_inherits_reasoning_split(self):
+        assert get_capabilities("MiniMax-M3-highspeed").requires_reasoning_split is True
+
+    def test_non_reasoning_minimax_does_not_get_reasoning_split(self):
+        # Coding Plan, MiniMax-Text-01, and any non-M2-prefixed MiniMax model
+        # reject the reasoning_split kwarg via the openai SDK's strict
+        # validation (#826). Default capability has it disabled.
+        for model in ("minimax-text-01", "MiniMax-Coding-Plan", "abab6.5-chat"):
+            assert get_capabilities(model).requires_reasoning_split is False
+
 
 @pytest.mark.unit
 class TestDefault:

tests/test_minimax.py

@@ -42,6 +42,18 @@ class TestMinimaxReasoningSplit:
         # the caller passed. setdefault leaves an existing value alone.
         assert payload.get("reasoning_split") in (False, True)
 
+    def test_non_reasoning_minimax_does_not_inject_reasoning_split(self):
+        """Coding Plan / MiniMax-Text-01 / any non-M2-prefixed model must NOT
+        receive reasoning_split — the openai SDK rejects unknown kwargs with
+        TypeError (#826)."""
+        for model in ("minimax-text-01", "MiniMax-Coding-Plan"):
+            payload = _client(model)._get_request_payload(
+                [HumanMessage(content="hi")]
+            )
+            assert "reasoning_split" not in payload, (
+                f"{model!r} payload unexpectedly contains reasoning_split"
+            )
+
 
 @pytest.mark.unit
 class TestMinimaxStructuredOutputDispatch: