feat(ts-sdk): allow overriding CLI environment (#6648)

## Summary
- add an `env` option for the TypeScript Codex client and plumb it into
`CodexExec` so the CLI can run without inheriting `process.env`
- extend the test spy to capture spawn environments, add coverage for
the new option, and document how to use it

## Testing
- `pnpm test` *(fails: corepack cannot download pnpm because outbound
network access is blocked in the sandbox)*

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6916b2d7c7548322a72d61d91a2dac85)

sdk/typescript/README.md

@@ -115,3 +115,19 @@ const thread = codex.startThread({
   skipGitRepoCheck: true,
 });
 ```
+
+### Controlling the Codex CLI environment
+
+By default, the Codex CLI inherits the Node.js process environment. Provide the optional `env` parameter when instantiating the
+`Codex` client to fully control which variables the CLI receives—useful for sandboxed hosts like Electron apps.
+
+```typescript
+const codex = new Codex({
+  env: {
+    PATH: "/usr/local/bin",
+  },
+});
+```
+
+The SDK still injects its required variables (such as `OPENAI_BASE_URL` and `CODEX_API_KEY`) on top of the environment you
+provide.