test: isolate app-server-client in-process test state (#21328)

## Why

The in-process `app-server-client` tests were still building their
configs from the ambient `codex_home` and letting the embedded app
server create its own state DB when `state_db` was absent. That matters
because in-process startup falls back to
`init_state_db_from_config(...)` in that case, so tests can otherwise
share persisted state instead of getting isolated fixtures:
[`app-server/src/in_process.rs`](https://github.com/openai/codex/blob/a98623511ba433154ec811fc63091617f5945438/codex-rs/app-server/src/in_process.rs#L368-L373).

## What changed

- Give each in-process test client its own temporary `codex_home`.
- Initialize the matching state DB from that per-client config and pass
it into the client explicitly.
- Keep the temp directory alive for the lifetime of the test client
through a small `TestClient` wrapper.
- Add `tempfile` as a dev dependency for the new harness.

The updated setup lives in
[`app-server-client/src/lib.rs`](https://github.com/openai/codex/blob/35c1133d45d10931914dbb88a1246a195d025ff6/codex-rs/app-server-client/src/lib.rs#L982-L1055).

## Testing

- Existing `codex-app-server-client` tests continue to exercise the
updated in-process client path through the isolated helper.

codex-rs/Cargo.lock

@@ -1944,6 +1944,7 @@ dependencies = [
  "pretty_assertions",
  "serde",
  "serde_json",
+ "tempfile",
  "tokio",
  "tokio-tungstenite",
  "toml 0.9.11+spec-1.1.0",