Session Store is a single point of failure for 7 modules, from the clients to the package exports

Seven modules depend directly on Session Store, with no fallback: ClaudeSDKClient, the Internal Client, Package Exports, Session Mutations, Session Resume, Sessions and the Transcript Mirror Batcher. Both clients call validate_session_store_options, Session Resume and Session Mutations call _store_implements, and Package Exports re-exports InMemorySessionStore. Package Exports also depends on ClaudeSDKClient, so a failure reaches the public entry point twice.

Impact: A defect in Session Store breaks both the interactive and one-shot clients and every session read, write and resume, and shows up at the public package surface.

Session Store is the hub of session management: 12 of the 23 other elements in this package depend on it, directly or indirectly. Treat changes to session_store.py and session_store_validation.py as changes to the whole session feature, and cover them with tests that go through both clients.