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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Thanks!

[rohityan]

Hi @stakeswky , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @xuanyang15 , can you please review this.

[xuanyang15]

@DeanChensj Could you please help review?

[adk-bot]

Thank you @stakeswky for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit 2780ae2.

Closing this PR as the changes are now in the main branch.