feat(sessions): add public prepare_tables() for eager table initialization

DatabaseSessionService currently initializes database tables lazily on
the first database operation. This causes a noticeable latency spike on
the first user request, which degrades user experience in production
applications.

Add a public `prepare_tables()` method that delegates to the existing
`_prepare_tables()` so that applications can call it at startup time
(e.g. during app initialization or health check) to pay the table-
creation cost upfront.

The lazy behavior is preserved as-is for backward compatibility.

Author: Ferran Pons Serra

src/google/adk/sessions/database_session_service.py

@@ -252,6 +252,17 @@ async def _with_session_lock(
         else:
           self._session_lock_ref_count[lock_key] = remaining
 
+  async def prepare_tables(self):
+    """Eagerly prepare database tables.
+
+    Call this method right after construction to pay the table-creation cost
+    upfront (e.g. during application startup) instead of on the first
+    database operation.  It is safe to call more than once and is also called
+    internally before every database operation, so using it is optional but
+    recommended for latency-sensitive applications.
+    """
+    await self._prepare_tables()
+
   async def _prepare_tables(self):
     """Ensure database tables are ready for use.
 

tests/unittests/sessions/test_session_service.py

@@ -1181,6 +1181,33 @@ async def test_prepare_tables_idempotent_after_creation():
     await service.close()
 
 
+@pytest.mark.asyncio
+async def test_public_prepare_tables_eager_initialization():
+  """Calling the public prepare_tables() eagerly initializes tables so that
+  the first real database operation does not pay the setup cost.
+  """
+  service = DatabaseSessionService('sqlite+aiosqlite:///:memory:')
+  try:
+    # Before calling prepare_tables, tables are not created.
+    assert not service._tables_created
+    assert service._db_schema_version is None
+
+    # Eagerly prepare tables via the public API.
+    await service.prepare_tables()
+
+    # Tables should now be ready.
+    assert service._tables_created
+    assert service._db_schema_version is not None
+
+    # Subsequent operations should work without any additional setup cost.
+    session = await service.create_session(
+        app_name='app', user_id='user', session_id='s1'
+    )
+    assert session.id == 's1'
+  finally:
+    await service.close()
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     'state_delta, expect_app_lock, expect_user_lock',