fix(Session): prevent V8 JIT corruption during cleanup on Alpine

Hold strong references to Session JS objects during DeleteAllSessions()
cleanup. This prevents GC from destroying Sessions while we iterate,
avoiding V8 JIT page corruption that manifested as SIGSEGV on Alpine/musl.

The bug: When Sessions were GC'd during Jest cleanup, their ObjectReference
destructors corrupted V8's JIT allocations. This only happened on Alpine
because musl has different GC timing during process exit.

The fix: Three-phase cleanup:
1. Create temporary ObjectReferences to all Sessions (prevents GC)
2. Clear SQLite sessions and session_ pointers
3. Clear database_ref_ for all Sessions (may trigger GC, but safe)
4. Release temporary refs - Sessions can be GC'd with empty database_ref_

Author: mceachen