fix(migrate): close SQLite connection and clean temp palace on exception

[arnoldwender]

What and Why

Two resource leaks in mempalace.migrate that bite hardest on Windows:

1. SQLite connection leak in extract_drawers_from_sqlite()

mempalace/migrate.py:55 opened a connection at the top of the function and only closed it on the happy path. Any exception during query/iteration leaked the handle. On Windows that holds a file lock on chroma.sqlite3 — the rest of the migration cannot shutil.copytree or os.replace the palace directory until the process exits. Users see a confusing PermissionError after a partial extraction.

2. Orphan temp dir in migrate()

mempalace/migrate.py:232 (now ~234) called tempfile.mkdtemp(prefix="mempalace_migrate_") with no try/finally. If anything between mkdtemp and the final os.replace(temp_palace, palace_path) raises (chromadb open failure, batch import failure, count mismatch, EXDEV fallback failure), the partial palace directory under the system temp root leaks forever. For a 50K-drawer palace this can be hundreds of MB the user has to find and clean by hand.

Fix

1. extract_drawers_from_sqlite — wrap the connection in contextlib.closing(sqlite3.connect(db_path)) so close() runs on every exit path. detect_chromadb_version already had a try/finally and is left alone.
2. migrate — wrap the temp-palace import-and-swap dance in try/finally. The cleanup checks os.path.exists(temp_palace) first; on the happy path os.replace consumes the temp dir so the guard makes cleanup a no-op.

Test plan

• test_extract_drawers_returns_drawers — happy path still works after the closing refactor (real SQLite fixture with the schema extract_drawers_from_sqlite reads)
• test_migrate_cleans_temp_palace_on_chromadb_failure — mkdtemp's directory is removed when ChromaBackend.get_or_create_collection raises, asserted via a tracking_mkdtemp that captures the path
• All 7 test_migrate.py tests pass
• Full suite: 1318 passed locally (2 pre-existing test_backends.py::test_pin_hnsw_threads* failures unrelated — chromadb configuration_json["hnsw"] schema drift, also fails on clean develop)