fix(fallback): handle timeout aborts

Co-authored-by: Mykyta Bozhenko <21245729+cheeeee@users.noreply.github.com>

src/agents/model-fallback.test.ts

@@ -281,6 +281,70 @@ describe("runWithModelFallback", () => {
     expect(run.mock.calls[1]?.[1]).toBe("claude-haiku-3-5");
   });
 
+  it("falls back on timeout abort errors", async () => {
+    const cfg = makeCfg();
+    const timeoutCause = Object.assign(new Error("request timed out"), { name: "TimeoutError" });
+    const run = vi
+      .fn()
+      .mockRejectedValueOnce(
+        Object.assign(new Error("aborted"), { name: "AbortError", cause: timeoutCause }),
+      )
+      .mockResolvedValueOnce("ok");
+
+    const result = await runWithModelFallback({
+      cfg,
+      provider: "openai",
+      model: "gpt-4.1-mini",
+      run,
+    });
+
+    expect(result.result).toBe("ok");
+    expect(run).toHaveBeenCalledTimes(2);
+    expect(run.mock.calls[1]?.[0]).toBe("anthropic");
+    expect(run.mock.calls[1]?.[1]).toBe("claude-haiku-3-5");
+  });
+
+  it("falls back on abort errors with timeout reasons", async () => {
+    const cfg = makeCfg();
+    const run = vi
+      .fn()
+      .mockRejectedValueOnce(
+        Object.assign(new Error("aborted"), { name: "AbortError", reason: "deadline exceeded" }),
+      )
+      .mockResolvedValueOnce("ok");
+
+    const result = await runWithModelFallback({
+      cfg,
+      provider: "openai",
+      model: "gpt-4.1-mini",
+      run,
+    });
+
+    expect(result.result).toBe("ok");
+    expect(run).toHaveBeenCalledTimes(2);
+    expect(run.mock.calls[1]?.[0]).toBe("anthropic");
+    expect(run.mock.calls[1]?.[1]).toBe("claude-haiku-3-5");
+  });
+
+  it("does not fall back on user aborts", async () => {
+    const cfg = makeCfg();
+    const run = vi
+      .fn()
+      .mockRejectedValueOnce(Object.assign(new Error("aborted"), { name: "AbortError" }))
+      .mockResolvedValueOnce("ok");
+
+    await expect(
+      runWithModelFallback({
+        cfg,
+        provider: "openai",
+        model: "gpt-4.1-mini",
+        run,
+      }),
+    ).rejects.toThrow("aborted");
+
+    expect(run).toHaveBeenCalledTimes(1);
+  });
+
   it("appends the configured primary as a last fallback", async () => {
     const cfg = makeCfg({
       agents: {