contents rearrange still raises on id-less / unmatched trailing function responses (follow-up to orphan FR prune)

[a2105z]

Describe the Bug

#6582 / 73e8625 added _drop_orphaned_function_responses, which correctly prunes id'd function responses with no matching function call before contents assembly.

Two gaps remain and can still permanently poison a session (raise happens in request preprocessing, before any user callback):

1. _rearrange_events_for_latest_function_response still raises ValueError: No function call event found for function responses ids: ... when it cannot pair the trailing FR event. @GWeale noted on #6587 that this rearrange-path drop is still worth having as defense in depth (main only prunes responses that carry an id).
2. Id-less / empty-id trailing FRs are intentionally skipped by the prune helper (if response.id), but then rearrange adds None / '' into the id set and raises — so sessions with stripped or missing FR ids still die on the next turn.

Related hole: when rearrange finds a matching FC event but the trailing FR event also carries unmatched ids (Last response event should only contain the responses...), it still raises instead of dropping the unmatched parts.

Steps to Reproduce

from google.adk.events.event import Event
from google.adk.flows.llm_flows.contents import _get_contents
from google.genai import types

events = [
  Event(author='user', content=types.Content(role='user', parts=[types.Part(text='hi')])),
  Event(author='agent', content=types.Content(role='model', parts=[types.Part(text='done')])),
  Event(
    author='agent',
    content=types.Content(
      role='user',
      parts=[types.Part(function_response=types.FunctionResponse(name='tool', id=None, response={'ok': True}))],
    ),
  ),
]
_get_contents(None, events, 'agent')  # ValueError: ... ids: {None}

Expected Behavior

Orphaned / unpairable trailing function responses are dropped (with a warning), contents assembly continues, and the session stays usable.

Observed Behavior

ValueError during contents assembly; every later turn replays the same fatal history.

Environment

• ADK: current main (post-73e8625)
• Related: #6582 (closed), #6587 (closed as duplicate of landed prune; rearrange follow-up requested)

Additional Context

Happy to send the rearrange-path follow-up PR referenced by @GWeale on #6587.

[a2105z]

Taking this — follow-up PR: #6752

[surajksharma07]

@a2105z The id=None trailing response slips past the orphan-prune's if response.id check and blows up in rearrange exactly as you traced.
Tried the same drop-unpairable-trailing-FRs approach and it resolved cleanly here, id-less pairs and unmatched-extras included.
#6752 looks like it's built the same way, could you confirm it holds against your original edge cases before this goes up for merge?

[a2105z]

@surajksharma07 Thanks for double-checking — confirmed locally against #6752.

Re-ran the original edge cases on the PR branch:

• trailing id=None after valid FC/FR history (the prune slip you called out)
• trailing empty-string id
• multiple consecutive id-less trailing FRs
• id’d orphan via the existing prune path
• rearrange defense-in-depth when prune is bypassed
• unmatched extras on a matched call event (strip, don’t raise)
• id-less adjacent FC/FR pair still kept (stripped-id model families)
• prior valid history preserved after dropping a trailing id-less orphan

All 8 held, plus pytest tests/unittests/flows/llm_flows/test_contents_function.py tests/unittests/flows/llm_flows/test_contents.py → 56 passed. No further changes needed on this branch from what I can see.