fix(vector_io): set WAL mode and busy_timeout on sqlite-vec connections

[extrasmall0]

The inline sqlite-vec provider opens database connections without setting WAL mode or busy_timeout. When running with multiple uvicorn workers (server.workers > 1), concurrent writes fail immediately with sqlite3.OperationalError: database is locked instead of waiting and retrying.

The SQL store backend (sqlalchemy_sqlstore.py) already handles this correctly by setting journal_mode=WAL, busy_timeout=5000, and synchronous=NORMAL. This patch applies the same pragmas to the shared _create_sqlite_connection() helper used by the vector store.

Changes:

• Set timeout=5.0 on sqlite3.connect() as a Python-level busy wait
• Execute PRAGMA journal_mode=WAL for concurrent reader/writer support
• Execute PRAGMA busy_timeout=5000 so SQLite retries instead of raising immediately
• Execute PRAGMA synchronous=NORMAL for better write throughput (still safe with WAL)
• Added a unit test verifying the pragmas are set on new connections

Fixes #5344

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[extrasmall0]

Quick status refresh after another recheck: this PR is still approved, and the core sqlite-vec behavior exercised by the patch remains the same. The practical change is still just enabling WAL mode and a busy timeout on the sqlite-vec connection so concurrent access is less likely to fail with transient locking issues. The branch now needs a rebase because main moved forward, but there is no new design question on the fix itself. If the maintainers still want this path, I can rebase and refresh it.