fix(openrouter): skip reasoning.effort injection for x-ai/grok models

x-ai/grok models on OpenRouter do not support the reasoning.effort
parameter and reject payloads containing it with "Invalid arguments
passed to the model." Skip reasoning injection for these models, the
same way we already skip it for the dynamic "auto" routing model.

Closes #32039

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/agents/pi-embedded-runner-extraparams.test.ts

@@ -317,6 +317,38 @@ describe("applyExtraParamsToAgent", () => {
     expect(payloads[0]).toEqual({ reasoning: { max_tokens: 256 } });
   });
 
+  it("does not inject reasoning.effort for x-ai/grok models on OpenRouter (#32039)", () => {
+    const payloads: Record<string, unknown>[] = [];
+    const baseStreamFn: StreamFn = (_model, _context, options) => {
+      const payload: Record<string, unknown> = {};
+      options?.onPayload?.(payload);
+      payloads.push(payload);
+      return {} as ReturnType<StreamFn>;
+    };
+    const agent = { streamFn: baseStreamFn };
+
+    applyExtraParamsToAgent(
+      agent,
+      undefined,
+      "openrouter",
+      "x-ai/grok-4.1-fast",
+      undefined,
+      "medium",
+    );
+
+    const model = {
+      api: "openai-completions",
+      provider: "openrouter",
+      id: "x-ai/grok-4.1-fast",
+    } as Model<"openai-completions">;
+    const context: Context = { messages: [] };
+    void agent.streamFn?.(model, context, {});
+
+    expect(payloads).toHaveLength(1);
+    expect(payloads[0]).not.toHaveProperty("reasoning");
+    expect(payloads[0]).not.toHaveProperty("reasoning_effort");
+  });
+
   it("normalizes thinking=off to null for SiliconFlow Pro models", () => {
     const payloads: Record<string, unknown>[] = [];
     const baseStreamFn: StreamFn = (_model, _context, options) => {