fix: enable SQLite foreign key enforcement in get_dialect()

[Tusm11]

Problem

SQLite disables foreign key constraint enforcement by default. This causes ON DELETE CASCADE constraints to be silently ignored, leaving orphaned rows in memori_entity_fact_mention table after delete_entity_memories() is called.

Example scenario:

1. Entity has facts stored
2. Mentions link facts to conversations
3. User calls delete_entity_memories(entity_id)
4. Facts get deleted -DONE
5. But mention rows stay behind (should cascade delete)

Solution

Execute PRAGMA foreign_keys = ON when SQLite dialect is detected in the get_dialect() method. This enables proper foreign key constraint enforcement, making cascade deletes work as intended.

Changes Made

• memori/storage/adapters/sqlalchemy/_adapter.py: Added PRAGMA statement for SQLite
• tests/storage/drivers/test_sqlite_driver.py: Tests verify the behavior

Testing

• All 26 SQLite driver tests pass
• Ran uv run pytest tests/storage/drivers/test_sqlite_driver.py -v
• No new issues or regressions

Impact

• Python SDK (memori/): Affected
• Storage adapters: Affected
• Breaking changes: None - backward compatible
• User-facing: Yes - fixes data integrity issue

[jayyao18]

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