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

[stakeswky]

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

# 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:

# 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.

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