fix(providers): wire MiMo thinking control on gateway providers (#3845)

The xiaomi_mimo ProviderSpec carries thinking_style="thinking_type", but
gateway providers (OpenRouter etc.) route MiMo under their own spec
which has no thinking_style. As a result, `reasoning_effort="none"` was
silently ignored: `{"thinking": {"type": "disabled"}}` was never
injected and responses still contained reasoning_content.

Mirror the Kimi pattern that already handles the same problem: add an
explicit _MIMO_THINKING_MODELS allowlist (mimo-v2.5-pro, mimo-v2.5,
mimo-v2-pro, mimo-v2-omni — per Xiaomi docs), an _is_mimo_thinking_model
helper that strips publisher prefixes ("xiaomi/mimo-v2.5-pro" matches),
and a sibling branch in _build_kwargs that injects the thinking payload
by model name. mimo-v2-flash is intentionally excluded — it has no
thinking mode.

Also include MiMo in the explicit_thinking predicate so the
reasoning_content backfill (#3554, #3584) covers the gateway path
consistently with the direct path.

Tests cover the gateway disable/enable signals, bare-slug fallback,
flash exclusion, and a non-MiMo sanity check.

tests/providers/test_xiaomi_mimo_thinking.py

@@ -31,6 +31,12 @@ def _mimo_spec():
     return specs["xiaomi_mimo"]
 
 
+def _openrouter_spec():
+    """Return the registered OpenRouter ProviderSpec (no thinking_style)."""
+    specs = {s.name: s for s in PROVIDERS}
+    return specs["openrouter"]
+
+
 def _mimo_provider() -> OpenAICompatProvider:
     return OpenAICompatProvider(
         api_key="test-key",
@@ -39,6 +45,15 @@ def _mimo_provider() -> OpenAICompatProvider:
     )
 
 
+def _openrouter_provider(default_model: str) -> OpenAICompatProvider:
+    """Provider configured as OpenRouter (gateway, no thinking_style on spec)."""
+    return OpenAICompatProvider(
+        api_key="sk-or-test",
+        default_model=default_model,
+        spec=_openrouter_spec(),
+    )
+
+
 def _simple_messages() -> list[dict[str, Any]]:
     return [{"role": "user", "content": "hello"}]
 
@@ -119,3 +134,69 @@ def test_mimo_reasoning_effort_unset_preserves_provider_default():
     )
     assert "reasoning_effort" not in kwargs
     assert "extra_body" not in kwargs
+
+
+# ---------------------------------------------------------------------------
+# Gateway path: MiMo routed through OpenRouter (no spec.thinking_style)
+# ---------------------------------------------------------------------------
+
+
+def test_mimo_via_openrouter_reasoning_effort_none_disables_thinking():
+    """OpenRouter routes MiMo as "xiaomi/mimo-v2.5-pro"; the openrouter spec
+    has no thinking_style, so the disable signal must come from the
+    model-name path (#3845)."""
+    provider = _openrouter_provider("xiaomi/mimo-v2.5-pro")
+    kwargs = provider._build_kwargs(
+        messages=_simple_messages(),
+        tools=None, model=None, max_tokens=100,
+        temperature=0.7, reasoning_effort="none", tool_choice=None,
+    )
+    assert "reasoning_effort" not in kwargs
+    assert kwargs["extra_body"] == {"thinking": {"type": "disabled"}}
+
+
+def test_mimo_via_openrouter_reasoning_effort_medium_enables_thinking():
+    """Same as the direct path: any non-none/minimal effort enables thinking."""
+    provider = _openrouter_provider("xiaomi/mimo-v2.5-pro")
+    kwargs = provider._build_kwargs(
+        messages=_simple_messages(),
+        tools=None, model=None, max_tokens=100,
+        temperature=0.7, reasoning_effort="medium", tool_choice=None,
+    )
+    assert kwargs.get("reasoning_effort") == "medium"
+    assert kwargs["extra_body"] == {"thinking": {"type": "enabled"}}
+
+
+def test_mimo_via_openrouter_bare_slug_also_matches():
+    """Bare "mimo-v2.5-pro" (no publisher prefix) must also match the
+    allowlist, since gateways sometimes accept either form."""
+    provider = _openrouter_provider("mimo-v2.5-pro")
+    kwargs = provider._build_kwargs(
+        messages=_simple_messages(),
+        tools=None, model=None, max_tokens=100,
+        temperature=0.7, reasoning_effort="none", tool_choice=None,
+    )
+    assert kwargs["extra_body"] == {"thinking": {"type": "disabled"}}
+
+
+def test_mimo_flash_via_openrouter_does_not_inject_thinking():
+    """mimo-v2-flash has no thinking mode per Xiaomi docs; the allowlist
+    excludes it, so no thinking field should be injected on the gateway path."""
+    provider = _openrouter_provider("xiaomi/mimo-v2-flash")
+    kwargs = provider._build_kwargs(
+        messages=_simple_messages(),
+        tools=None, model=None, max_tokens=100,
+        temperature=0.7, reasoning_effort="none", tool_choice=None,
+    )
+    assert "extra_body" not in kwargs
+
+
+def test_non_mimo_model_via_openrouter_unaffected():
+    """Sanity: a non-MiMo, non-Kimi model through OpenRouter is untouched."""
+    provider = _openrouter_provider("openai/gpt-4o")
+    kwargs = provider._build_kwargs(
+        messages=_simple_messages(),
+        tools=None, model=None, max_tokens=100,
+        temperature=0.7, reasoning_effort="none", tool_choice=None,
+    )
+    assert "extra_body" not in kwargs