test(codex-local): regression for CodexRpcClient spawn ENOENT

Add a Vitest case that mocks `node:child_process.spawn` so the child
emits `error` (ENOENT) after the constructor attaches listeners.
`getQuotaWindows()` must resolve with `ok: false` instead of leaving an
unhandled `error` event on the process.

Register `packages/adapters/codex-local` in the root Vitest workspace.

Document in DEVELOPING.md that a missing `codex` binary should not take
down the API server during quota polling.

packages/adapters/codex-local/vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    environment: "node",
+  },
+});