fix: resolve review_code chroma init race

[gdwrd]

Summary

This PR fixes a race condition during review_code query-engine initialization, especially with Chroma in non-interactive flows after indexing.
The issue could trigger initialization conflicts (including default_tenant connection errors) under parallel review workers.

Root Cause

MetisEngine._init_and_get_query_engines() and vector-store init/close paths were not serialized, so concurrent workers could race through
backend init and teardown.

Changes

• Added engine-level synchronization:
  • MetisEngine now uses a lock (_qe_init_lock) around query-engine init and close().
  • Ensures backend init/query-engine creation happens once per lifecycle.
• Hardened Chroma backend lifecycle:
  • Added lock-protected init()/close().
  • Added explicit state reset helper and client shutdown helper.
  • On init failure, stops partially created client, clears state, and raises VectorStoreInitError with exception chaining.
• Hardened PGVector backend lifecycle:
  • Added lock-protected init()/close().
  • Centralized cleanup to close stores, dispose engines, and clear storage contexts.
  • On partial init failure, performs cleanup and raises VectorStoreInitError with exception chaining.
• Expanded test coverage:
  • Thread-safe single-init behavior for engine/chroma/pgvector.
  • Retryability after init failures.
  • Correct cache clearing on close (including backend close failure path).
  • Repro/guard test for index -> non-interactive review_code race.

Validation

Executed:

UV_CACHE_DIR=$PWD/.uv-cache uv run pytest tests/test_engine_chroma.py tests/test_engine_core.py tests/test_engine_review.py tests/
test_pg_backend_mocked_unit.py

Result:

• 23 passed, 1 skipped in 3.54s
• Skipped test is Postgres-gated (--postgres not enabled).

[mk-arm]

I think we can move this outside of the lock as _clear_state() has already setself._client = None and self._initialized = False and it's now safe for another concurrent init()

[mk-arm]

Similarly if we split this function, we can clear state under lock and do the slower engine disposal outside of it

[mpekatsoula]

Closing in favor of: #171