fix: select latest SQLite checkpoint when timestamps tie

[Kuang-xianxin]

SQLitePersister.load(partition_key, None) and its async counterpart can return an older checkpoint when several saves share a timestamp. SQLite's default CURRENT_TIMESTAMP has second precision, and the current query orders only by that timestamp. In a deterministic reproduction, saving sequence 11 followed by sequence 12 and loading after reopening the database returns sequence 11. Across applications it can also return the wrong app.

Changes

• Use descending rowid as the secondary ordering key in the latest-across-apps query, after created_at.
• Apply the same fix to synchronous SQLite and aiosqlite persistence.
• Add deterministic regressions for same-app and cross-app saves, default and explicit partitions, partition isolation, database reopening, and explicit historical lookup. A separate control verifies that timestamps remain the primary ordering key.

This uses the rowid already present in tables created by these persisters; no schema migration or change to app-specific sequence selection is needed. The tie-breaker follows insertion order in the persister-managed table, not a portable identifier for externally rewritten database rows.

Related: #807 explicitly notes the timestamp-tie limitation in its test-only PR. This PR fixes the behavior in both implementations; it does not duplicate those construction/lifecycle tests or depend on that PR.

How I tested this

Python 3.13.14 on Windows, using actual SQLite/aiosqlite databases:

python -m pytest tests/core/test_persistence.py tests/integrations/persisters/test_b_aiosqlite.py -q -k load_latest
Before the fix: 8 failed, 2 passed

python -m pytest tests/core/test_persistence.py tests/integrations/persisters/test_b_aiosqlite.py tests/core/test_application.py -q --timeout=20
After the fix: 199 passed

Pre-commit passes on all four changed files: Black 23.11.0, isort 5.12.0, flake8 6.1.0, whitespace/EOF, AST, and ASF headers. The frontend hook was explicitly skipped because this changes only Python persistence; the full external-database test matrix was not run.

Notes

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal
• Applicable pre-commit checks pass (frontend hook excluded as described above)
• Any change in functionality is tested
• Existing database format and explicit historical lookup are preserved