fix(memory): atomic cross-process MySQL checkpoint index (resolves #301)

[fede-kamel]

Summary

Resolves #301. Makes the MySQL checkpoint index updates atomic across processes, closing the cross-process race that #300's per-instance lock could not cover.

Stacked on #303 (base branch = fix/mysql-pool-idle-in-transaction). The new integration test's DROP TABLE teardown needs #303's rollback-on-release fix to avoid the idle-in-transaction MDL hang. Retarget to main once #303 merges.

The bug

StorageBackendAdapter keeps the {thread}:_checkpoints index as a blob and updates it with a load-modify-save guarded only by a per-instance asyncio.Lock (#300). That serializes writers within one process, but two processes / adapter instances over a shared store hold independent locks, so the read-modify-write interleaves and drops index entries. The checkpoint data keys are never lost (distinct keys, no RMW) — only the index under-reports, so list_checkpoints / time-travel miss checkpoints.

Reproduced against MySQL 9.6 (20 concurrent saves to one thread):

one-instance (shared lock)     = 20, 20, 20   ✅  (#300 in-process fix works)
two-instances (separate locks) = 11, 10, 10   ❌  (cross-process loss)

The fix

Give the MySQL backend atomic index primitives and have the adapter delegate to them when present (other backends keep the lock+blob fallback unchanged):

• index_add — single INSERT … ON DUPLICATE KEY UPDATE using JSON_ARRAY_INSERT(data, '$.checkpoints[0]', …). The append is serialized by InnoDB's row lock, so concurrent cross-process writers can't clobber each other. Keeps the adapter's {"checkpoints": [...]} shape, newest-first.
• index_remove — SELECT … FOR UPDATE + rewrite, so the index RMW is row-locked across processes.
• list_checkpoints — de-duplicates by checkpoint_id at read time (the atomic append doesn't de-dup a re-saved id).
• _async_backend_op — capability detection requires a real coroutine method, so MagicMock test doubles aren't mistaken for the atomic-index backend.

After the fix: two-instances is 20/20.

Validation

• New integration test test_mysql_adapter_cross_process_index_no_loss (two adapter instances, one table): all 20 entries retained.
• MySQL integration suite: 12 passed against MySQL 9.6.
• Full unit suite green; coverage 93.3% (≥90% gate). mysql.py 97%, adapters.py 99%. Added unit tests for index_add/index_remove (atomic SQL + FOR UPDATE + missing-row no-op) and adapter delegation incl. the MagicMock-safety guard.
• ruff, ruff format --check, hatch run typecheck all clean.