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_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: