Make normalizeSpawn truly async to prevent event loop blocking

[Copilot]

The normalizeSpawn function in launch.ts returned a synchronous wrapper that called spawnSync, blocking the event loop during async runLaunch preflight checks.

Changes

• Replaced spawnSync with async spawn — Added spawnAsync helper that wraps child_process.spawn in a Promise, collecting stdout/stderr via streams without blocking
• Updated type signatures — Changed DoctorDependencies.spawnCommand to return Promise<...>
• Awaited all spawn calls — Added await to preflight check, codex auth, and MCP registration flows in both launch.ts and doctor.ts
• Updated test mocks — Made all spawnCommand test doubles async

Implementation

The spawnAsync wrapper always resolves (never rejects) to match spawnSync API contract, communicating errors via status code and stderr:

function spawnAsync(
  command: string,
  args: string[],
  options: { cwd: string; encoding: BufferEncoding }
): Promise<Pick<SpawnSyncReturns<string>, 'status' | 'stdout' | 'stderr'>> {
  return new Promise((resolve) => {
    const child = spawn(command, args, { cwd: options.cwd, stdio: ['ignore', 'pipe', 'pipe'] });
    // ... collect stdout/stderr asynchronously
    child.on('close', (code) => resolve({ status: code ?? 0, stdout, stderr }));
    child.on('error', (error) => resolve({ status: 1, stdout, stderr: error.message }));
  });
}

This preserves existing error handling patterns while eliminating event loop blocking.

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