Add regression test for write-after-endInput race in ProcessTransport

[dumko2001]

Summary

Regression test for #148. Reproduces the race where `handleControlRequest()` is launched fire-and-forget in `readMessages()`. When a `result` message arrives immediately after a `control_request`, `endInput()` closes stdin before the async handler finishes (e.g. `canUseTool` delay). The handler then calls `write()` on the ended stream — throwing `ERR_STREAM_WRITE_AFTER_END`. The `catch` block attempts a second `write()`, which becomes an unhandled rejection and crashes Node.js.

The test currently fails against the published SDK, confirming the bug. It will pass once the fix is applied.

Fix (for SDK source)

// ProcessTransport class:
private stdinEnded = false;

endInput(): void {
  this.stdinEnded = true; // set before .end() so concurrent writes see it immediately
  if (this.processStdin) this.processStdin.end();
}

write(data: string): void {
  if (this.stdinEnded) return; // first guard, silent drop, never throws
  // ... rest unchanged
}

Silent drop is safe — after endInput() the subprocess has already emitted its result and will not read further stdin.

Changes

• tests/process-transport-write-after-endinput.test.ts — regression test using spawnClaudeCodeProcess to inject a controlled mock process, with a canUseTool delay that guarantees the race fires
• package.json, tsconfig.json, vitest.config.ts, .gitignore — minimum Vitest infrastructure (not present on main)

Closes #148