feat(sessions): add a client-set title to sessions

[answersamir]

Summary

Adds an optional, client-set title to the Session model and threads it through every built-in session service and the web server, so frontends can show readable titles in session/chat-history lists instead of UUIDs.

This is the manual / client-set variant of #1340 (the service stores a title the caller provides; ADK does not auto-generate it), and builds on the approach of the closed #3703.

What's included

• Model: Session.title (nullable, defaults to None); surfaced by get_session and list_sessions.
• Service interface: BaseSessionService.update_session_title(...) as a concrete method that raises NotImplementedError by default — mirroring get_user_state, so existing third-party BaseSessionService subclasses keep working without changes (no new abstract method).
• Implementations: InMemorySessionService, DatabaseSessionService and SqliteSessionService implement it. VertexAiSessionService raises NotImplementedError, because the managed Agent Engine Session API exposes no client-set title (same precedent as its get_user_state).
• HTTP: the existing PATCH /apps/{app_name}/users/{user_id}/sessions/{session_id} endpoint accepts an optional title (and state_delta is now optional), so a title can be set with or without a state change — no new route.

Existing-database handling (no schema-version bump)

title is a nullable, additive column, so this does not bump the DB schema version. Because metadata.create_all() never adds a column to an existing table, DatabaseSessionService gains a small additive column reconciler, _ensure_schema_columns_exist, that mirrors the existing _ensure_schema_indexes_exist index healer — an existing database gains the title column in place on the next connect (each ALTER guarded by a SAVEPOINT so concurrent instances don't poison the setup transaction). SqliteSessionService does the same via ALTER TABLE ... ADD COLUMN. Legacy v0 databases are left untouched.

Behavior note

Setting a title preserves the session's update_time — a title is display metadata and must not invalidate an in-flight copy of the session, which would otherwise trip the stale-writer check on the next append_event.

Tests

• Parametrized update_session_title round-trip, not-found, and "preserves update_time" tests across all four SessionServiceTypes (in-memory, in-memory light-copy, database, sqlite).
• VertexAiSessionService raises NotImplementedError.
• Database and Sqlite column-healer tests (an existing DB predating title gains the column; a v0 DB is untouched).
• Web-server PATCH-with-title and title-only tests.

The full tests/unittests suite passes locally (9567 passed); pyink / isort / ruff / compliance pre-commit checks are clean.

Related

• Addresses Add Session title #1340.
• Builds on the (closed) feat(sessions): Add session title management functionality #3703 — thanks to @zhangp365 for the original approach.