fix: temp-scoped state now visible to subsequent agents in same invocation

Merge https://github.com/google/adk-python/pull/4618

## Summary

Fixes #4564

When using `output_key` with a `temp:` prefix (e.g. `output_key='temp:result'`) in a `SequentialAgent`, the output was silently lost. Agent-2 could never read the temp state written by agent-1.

## Root Cause

Two issues in `append_event`:

1. `_trim_temp_delta_state()` removed temp keys from the event delta **before** `_update_session_state()` could apply them to the in-memory session
2. `_update_session_state()` also explicitly skipped `temp:`-prefixed keys

```python
# Before (broken ordering):
async def append_event(self, session, event):
    event = self._trim_temp_delta_state(event)   # temp keys gone!
    self._update_session_state(session, event)    # nothing to apply
```

## Fix

Introduce `_apply_temp_state()` which writes temp-scoped keys to the in-memory `session.state` **before** the event delta is trimmed:

```python
# After:
async def append_event(self, session, event):
    self._apply_temp_state(session, event)        # temp keys → session.state
    event = self._trim_temp_delta_state(event)    # temp keys removed from delta
    self._update_session_state(session, event)    # non-temp keys applied
```

This ensures:
- ✅ Temp state is available to subsequent agents within the same invocation
- ✅ Temp state is still stripped from event deltas (not persisted to storage)
- ✅ All three session services (InMemory, Database, SQLite) behave consistently

## Files Changed

- `src/google/adk/sessions/base_session_service.py`: Added `_apply_temp_state()`, reordered `append_event` logic, removed temp-skip in `_update_session_state`
- `src/google/adk/sessions/database_session_service.py`: Added `_apply_temp_state()` call before trim
- `src/google/adk/sessions/sqlite_session_service.py`: Added `_apply_temp_state()` call before trim
- `tests/unittests/sessions/test_session_service.py`: Updated existing test + added new test for sequential agent scenario

## Testing

All 67 session service tests pass across InMemory, Database, and SQLite backends.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4618 from stakeswky:fix/temp-state-output-key b9fc737e7a6dc07e06e99af3271a8fc026acae4a
PiperOrigin-RevId: 878499263

src/google/adk/sessions/base_session_service.py

@@ -106,13 +106,35 @@ class BaseSessionService(abc.ABC):
     """Appends an event to a session object."""
     if event.partial:
       return event
+    # Apply temp-scoped state to the in-memory session BEFORE trimming the
+    # event delta, so that subsequent agents within the same invocation can
+    # read temp values (e.g. output_key='temp:my_key' in SequentialAgent).
+    self._apply_temp_state(session, event)
     event = self._trim_temp_delta_state(event)
     self._update_session_state(session, event)
     session.events.append(event)
     return event
 
+  def _apply_temp_state(self, session: Session, event: Event) -> None:
+    """Applies temp-scoped state delta to the in-memory session state.
+
+    Temp state is ephemeral: it lives in the session's in-memory state for
+    the duration of the current invocation but is NOT persisted to storage
+    (the event delta is trimmed separately by _trim_temp_delta_state).
+    """
+    if not event.actions or not event.actions.state_delta:
+      return
+    for key, value in event.actions.state_delta.items():
+      if key.startswith(State.TEMP_PREFIX):
+        session.state[key] = value
+
   def _trim_temp_delta_state(self, event: Event) -> Event:
-    """Removes temporary state delta keys from the event."""
+    """Removes temporary state delta keys from the event.
+
+    This prevents temp-scoped state from being persisted, while the
+    in-memory session state (updated by _apply_temp_state) retains the
+    values for the duration of the current invocation.
+    """
     if not event.actions or not event.actions.state_delta:
       return event
 
@@ -128,6 +150,4 @@ class BaseSessionService(abc.ABC):
     if not event.actions or not event.actions.state_delta:
       return
     for key, value in event.actions.state_delta.items():
-      if key.startswith(State.TEMP_PREFIX):
-        continue
       session.state.update({key: value})

src/google/adk/sessions/database_session_service.py

@@ -522,6 +522,9 @@ class DatabaseSessionService(BaseSessionService):
     if event.partial:
       return event
 
+    # Apply temp state to in-memory session before trimming, so that
+    # subsequent agents within the same invocation can read temp values.
+    self._apply_temp_state(session, event)
     # Trim temp state before persisting
     event = self._trim_temp_delta_state(event)
 

src/google/adk/sessions/sqlite_session_service.py

@@ -361,6 +361,9 @@ class SqliteSessionService(BaseSessionService):
     if event.partial:
       return event
 
+    # Apply temp state to in-memory session before trimming, so that
+    # subsequent agents within the same invocation can read temp values.
+    self._apply_temp_state(session, event)
     # Trim temp state before persisting
     event = self._trim_temp_delta_state(event)
     event_timestamp = event.timestamp

tests/unittests/sessions/test_session_service.py

@@ -418,16 +418,41 @@ async def test_temp_state_is_not_persisted_in_state_or_events(session_service):
   )
   await session_service.append_event(session=session, event=event)
 
-  # Refetch session and check state and event
-  session_got = await session_service.get_session(
-      app_name=app_name, user_id=user_id, session_id='s1'
-  )
-  # Check session state does not contain temp keys
-  assert session_got.state.get('sk') == 'v2'
-  assert 'temp:k1' not in session_got.state
+  # Temp state IS available in the in-memory session (same invocation)
+  assert session.state.get('temp:k1') == 'v1'
+  assert session.state.get('sk') == 'v2'
+
   # Check event as stored in session does not contain temp keys in state_delta
-  assert 'temp:k1' not in session_got.events[0].actions.state_delta
-  assert session_got.events[0].actions.state_delta.get('sk') == 'v2'
+  assert 'temp:k1' not in event.actions.state_delta
+  assert event.actions.state_delta.get('sk') == 'v2'
+
+
+@pytest.mark.asyncio
+async def test_temp_state_visible_across_sequential_events(session_service):
+  """Temp state set by one event should be readable before the next event.
+
+  This simulates a SequentialAgent where agent-1 writes output_key='temp:out'
+  and agent-2 needs to read it from session.state within the same invocation.
+  """
+  app_name = 'my_app'
+  user_id = 'u1'
+  session = await session_service.create_session(
+      app_name=app_name, user_id=user_id, session_id='s_seq'
+  )
+
+  # Agent-1 writes temp state
+  event1 = Event(
+      invocation_id='inv1',
+      author='agent1',
+      actions=EventActions(state_delta={'temp:output': 'result_from_a1'}),
+  )
+  await session_service.append_event(session=session, event=event1)
+
+  # Agent-2 should be able to read temp state from the same session object
+  assert session.state.get('temp:output') == 'result_from_a1'
+
+  # But the event delta should NOT contain the temp key (not persisted)
+  assert 'temp:output' not in event1.actions.state_delta
 
 
 @pytest.mark.asyncio