fix(utils): report an unknown session id in the cache performance analyzer

Both analyzer entry points looked a session up and read its events without
checking the lookup succeeded, so an unknown session id raised AttributeError
on None rather than naming the session. They now raise ValueError.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 974117700

Author: GWeale

src/google/adk/utils/cache_performance_analyzer.py

@@ -60,6 +60,8 @@ async def _get_agent_cache_history(
         app_name=app_name,
         user_id=user_id,
     )
+    if session is None:
+      raise ValueError(f"Session not found: {session_id}")
     cache_history = []
 
     for event in session.events:
@@ -111,6 +113,8 @@ async def analyze_agent_cache_performance(
         app_name=app_name,
         user_id=user_id,
     )
+    if session is None:
+      raise ValueError(f"Session not found: {session_id}")
 
     # Collect token metrics from events
     total_prompt_tokens = 0

tests/unittests/utils/test_cache_performance_analyzer.py

@@ -90,6 +90,15 @@ async def test_get_agent_cache_history_empty_session(self):
 
     assert result == []
 
+  async def test_get_agent_cache_history_missing_session(self):
+    """An unknown session id is reported, not dereferenced."""
+    self.mock_session_service.get_session = AsyncMock(return_value=None)
+
+    with pytest.raises(ValueError, match="Session not found: no_such_session"):
+      await self.analyzer._get_agent_cache_history(
+          "no_such_session", "test_user", "test_app", "test_agent"
+      )
+
   async def test_get_agent_cache_history_no_cache_events(self):
     """Test getting cache history when no events have cache metadata."""
     events = [