fix(rpc): reject pending requests on child process exit

closes #4764

packages/coding-agent/test/rpc-client-process-exit.test.ts

@@ -0,0 +1,38 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, describe, expect, test } from "vitest";
+import { RpcClient } from "../src/modes/rpc/rpc-client.ts";
+
+const tempDirs: string[] = [];
+
+function writeChildScript(contents: string): string {
+	const dir = mkdtempSync(join(tmpdir(), "pi-rpc-client-exit-"));
+	tempDirs.push(dir);
+	const path = join(dir, "child.mjs");
+	writeFileSync(path, contents);
+	return path;
+}
+
+afterEach(() => {
+	for (const dir of tempDirs.splice(0)) {
+		rmSync(dir, { recursive: true, force: true });
+	}
+});
+
+describe("RpcClient child process failures", () => {
+	test("rejects an in-flight request when the child process exits", async () => {
+		const client = new RpcClient({
+			cliPath: writeChildScript(`
+process.stdin.once("data", () => {
+	process.exit(43);
+});
+process.stdin.resume();
+`),
+		});
+
+		await client.start();
+
+		await expect(client.getCommands()).rejects.toThrow(/Agent process exited \(code=43 signal=null\)/);
+	});
+});