test: cover SQLitePersister construction, lifecycle, and load variants

[adawang1210]

Summary

Coverage analysis showed several public SQLitePersister paths had no tests: from_values, from_config, copy(), the context-manager protocol, pickling (__getstate__/__setstate__, which the parallelism machinery relies on to ship persisters across processes), and the load() branches for app_id=None (latest across apps) and explicit sequence_id. This adds focused unit tests for each; no behavior changes.

Changes made

• tests/core/test_persistence.py: 7 new tests covering constructor classmethods, context-manager connection closing, copy() connection independence, pickle round-trip reconnection, and the remaining load() query branches (including a missing sequence_id returning None).

How to test

pytest tests/core/test_persistence.py

Notes

The app_id=None test asserts shape rather than which app wins: created_at has second resolution in SQLite, so latest-across-apps ordering is non-deterministic for rapid successive writes (asserting a specific winner would be flaky).

Checklist

• Tests added or updated
• All tests pass locally (pytest tests/ --ignore=tests/integration_tests: 580 passed, 5 skipped)
• Commits signed off (git commit -s)
• Pre-commit hooks pass (black, isort, flake8)
• CONTRIBUTING.md guidelines followed