fix(supervisor): suppress unhandled stream errors on child stdout/stderr (#99802)

* fix(supervisor): suppress unhandled stream errors on child stdout/stderr

Child process stdout and stderr streams can emit 'error' events
(EPIPE, resource limits, I/O errors) that crash the process when
no listener is attached. Add noop error handlers to prevent crashes
while preserving normal data/end/close event flow.

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

* test(supervisor): add regression test for stream error suppression

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

* fix(supervisor): guard child stream errors during setup

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/process/supervisor/adapters/child.test.ts

@@ -511,4 +511,42 @@ describe("createChildAdapter", () => {
     expect(first).toHaveBeenCalledWith("first");
     expect(second).toHaveBeenCalledWith("second");
   });
+
+  it("guards stream errors before output listeners are registered", async () => {
+    vi.useFakeTimers();
+    setPlatform("win32");
+    const { child, emitExit } = createStubChild(6666);
+    spawnWithFallbackMock.mockResolvedValue({
+      child,
+      usedFallback: false,
+    });
+    const adapter = await createChildAdapter({
+      argv: ["node", "-e", "setTimeout(() => {}, 1000)"],
+      stdinMode: "pipe-open",
+    });
+
+    const stdoutErr = new Error("simulated stdout pipe error");
+    const stderrErr = new Error("simulated stderr pipe error");
+    const settled = vi.fn();
+    void adapter.wait().then(settled);
+
+    emitExit(0, null);
+    expect(() => child.stdout?.emit("error", stdoutErr)).not.toThrow();
+    expect(() => child.stderr?.emit("error", stderrErr)).not.toThrow();
+    await vi.advanceTimersByTimeAsync(300);
+    expect(settled).not.toHaveBeenCalled();
+
+    adapter.onStdout(() => {});
+    adapter.onStdout(() => {});
+    adapter.onStderr(() => {});
+    adapter.onStderr(() => {});
+
+    expect(child.stdout?.listenerCount("error")).toBe(1);
+    expect(child.stderr?.listenerCount("error")).toBe(1);
+
+    child.stdout?.emit("close");
+    child.stderr?.emit("close");
+    await vi.advanceTimersByTimeAsync(0);
+    expect(settled).toHaveBeenCalledWith({ code: 0, signal: null });
+  });
 });

src/process/supervisor/adapters/child.ts

@@ -103,6 +103,11 @@ export async function createChildAdapter(params: {
   });
 
   const child = spawned.child as ChildProcessWithoutNullStreams;
+  // Pipe errors can arrive before output subscribers attach. Close remains
+  // responsible for decoder flush and Windows drain completion.
+  const ignoreOutputStreamError = () => {};
+  child.stdout.on("error", ignoreOutputStreamError);
+  child.stderr.on("error", ignoreOutputStreamError);
   const childStdin = spawned.child.stdin;
   let stdinDestroyed = childStdin?.destroyed ?? false;
   let stdinEnded = childStdin?.writableEnded === true || childStdin?.writableFinished === true;