test(app-server): run plain tests in process

[starr-openai]

Summary

• add a lazy in-process AppServerTestProcess path that starts the embedded app-server at initialize time
• preserve request submission order for embedded JSON-RPC requests
• move wrapper-gap app-server tests from child stdio processes to the embedded runtime

Scope

This intentionally keeps tests on child AppServerTestProcess when they exercise process/startup/network/lifecycle behavior, depend on process-global originator isolation, or assert rpc_transport == "stdio".

Current constructor count in codex-rs/app-server/tests: 126 in-process, 219 child-process.

Validation

• just fmt
• git diff --check
• devbox focused target:
  bazel test --bes_backend= --bes_results_url= --remote_cache= --experimental_remote_downloader= --remote_executor= --nocache_test_results --test_output=errors //codex-rs/app-server:app-server-all-test

The focused devbox run exercised the target after fixing a branch-caused in-process fuzzy-file-search request ordering failure. The remaining failure was in unchanged child-process account refresh tests (external_auth_refresh_error_fails_turn / external_auth_refresh_invalid_access_token_fails_turn), so this draft leaves that as residual unrelated flake/timing noise rather than widening the branch.