fix(rpc): surface drained stderr when startup fails under load

- RpcClient: when stdout closes before ready, race child.exited (which settles only after ptree drains the stderr tail for nonzero exits) for 250ms before rejecting, so the earlier-registered exit watcher rejects with the real stderr text instead of an empty 'Stderr:'.
- mock-rpc-agent fixture: await the stderr pipe write callback before process.exit so failure text cannot be dropped unflushed.
- sdk-tool-activation: restore the default extension-handler budget once the intentionally stalled activation times out, so full-suite machine load cannot also time out the genuine recovery registration.
Both tests flaked only under concurrent full-suite chunk load; 10 concurrent stress runs pass post-fix.

packages/coding-agent/src/modes/rpc/rpc-client.ts

@@ -353,6 +353,13 @@ export class RpcClient {
 			// failures are reaped by the readyPromise catch below; established
 			// workers are reaped here so pending requests cannot hang indefinitely.
 			if (!readySettled) {
+				// Stdout can close before the exit reaper finishes draining stderr.
+				// child.exited settles only after the stderr tail is complete (for
+				// nonzero exits), so give it a bounded head start: the exit watcher
+				// below was registered first and rejects with the real stderr text
+				// instead of an empty "Stderr:" (flaked under full-suite load).
+				await Promise.race([child.exited.catch(() => {}), Bun.sleep(250)]);
+				if (readySettled) return;
 				readySettled = true;
 				readyReject(new Error(`Agent output stream ended before ready. Stderr: ${child.peekStderr()}`));
 				return;

packages/coding-agent/test/fixtures/mock-rpc-agent.ts

@@ -30,7 +30,14 @@ const legacyState = {
 };
 
 if (Bun.env.MOCK_RPC_EXIT_BEFORE_READY) {
-	process.stderr.write(Bun.env.MOCK_RPC_EXIT_STDERR ?? "");
+	const message = Bun.env.MOCK_RPC_EXIT_STDERR ?? "";
+	if (message) {
+		// Await the pipe write: exiting immediately can drop unflushed stderr
+		// bytes, leaving the client's startup error without the failure text.
+		const { promise, resolve } = Promise.withResolvers<void>();
+		process.stderr.write(message, () => resolve());
+		await promise;
+	}
 	process.exit(Number(Bun.env.MOCK_RPC_EXIT_BEFORE_READY));
 }
 

packages/coding-agent/test/sdk-tool-activation.test.ts

@@ -1247,6 +1247,11 @@ describe("createAgentSession defaultInactive tool activation", () => {
 			const errors: string[] = [];
 			const unsubscribe = runner.onError(error => {
 				errors.push(error.error);
+				// The 10ms budget exists only to reap the stalled first handler
+				// quickly; handlers run sequentially and the budget is read per
+				// handler, so restoring it here keeps machine load from timing out
+				// the genuine recovery registration too (flaked in full-suite runs).
+				testSetExtensionHandlerTimeoutMs(EXTENSION_HANDLER_TIMEOUT_MS);
 			});
 			testSetExtensionHandlerTimeoutMs(10);
 
@@ -1454,6 +1459,10 @@ describe("createAgentSession defaultInactive tool activation", () => {
 
 			releaseStalledRegistration.resolve();
 			const failure = await detachedFailure.promise;
+			// Restore the default budget before the recovered registration flush:
+			// the 10ms budget was only for reaping the stalled activation, and the
+			// real presentation pass can exceed it under full-suite load.
+			testSetExtensionHandlerTimeoutMs(EXTENSION_HANDLER_TIMEOUT_MS);
 			releaseRecoveredRegistration.resolve();
 			await recoveredActivation.promise;