Fix empty rollout path app-server handling

[wiltzius-openai]

Summary

• Coerce path: "" to None at the v2 protocol params deserialization boundary for thread/resume and thread/fork.
• Restore the pre-ThreadStore running-thread resume behavior: if threadId is already running, rejoin it by id and treat a non-empty path only as a consistency check; otherwise cold resume keeps history > path > threadId precedence.
• Add protocol, resume, and fork regression coverage for empty path payloads; refresh app-server schema fixtures for the clarified params docs.

Tests

• just fmt
• just write-app-server-schema
• cargo test -p codex-app-server-protocol thread_path_params_deserialize_empty_path_as_none
• cargo test -p codex-app-server-protocol --test schema_fixtures
• cargo test -p codex-app-server empty_path
• RUST_MIN_STACK=8388608 cargo test -p codex-app-server --test all thread_resume_rejects_mismatched_path_for_running_thread_id
• RUST_MIN_STACK=8388608 cargo test -p codex-app-server --test all thread_resume_uses_path_over_non_running_thread_id