Fix SQLite threading bugs (#553)

Fixes two SQLite threading bugs that caused intermittent test failures
with errors like "cannot commit transaction - SQL statements in
progress" and "System database accessed before DBOS was launched".

## Changes

- Pass `engine_kwargs` to `create_engine` in
`SQLiteSystemDatabase._create_engine`, filtering PostgreSQL-specific
`connect_args` (`application_name`, `connect_timeout`)
- Reorder `_destroy()` to join background threads before closing
database connections
- Add 10-second timeout to thread joins with warning log if threads
don't exit
- Set `_run_background_processes = False` before joining threads to
signal graceful shutdown

## Testing

- Added `test_sqlite_engine_kwargs_passed` to verify `engine_kwargs` are
passed through
- Added `test_sqlite_filters_postgres_connect_args` to verify
PostgreSQL-specific args are filtered
- Added `test_destroy_closes_db_before_joining_threads` to reproduce the
race condition

## References

- https://github.com/pydantic/pydantic-ai/issues/3934

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Co-authored-by: Peter Kraft <petereliaskraft@gmail.com>