fix(flows): pair every function call with a result in request contents

[QuentinBisson]

Link to Issue or Description of Change

• Same failure as Orphaned function_call blocks from user interruption cause 400 errors #5856 (stale-closed), Persistent crash loop caused by missing tool_result in conversation history after interrupted execution #3971 and contents request-processor (_rearrange_events_for_latest_function_response) raises ValueError on an orphaned function response, permanently poisoning the session — raise happens before before_model_callback, so no user hook can intercept it #6582.

Problem

A function call and its result are two separate session events. When a turn ends between them (a restart, an OOM kill, a client disconnect, a cancellation), the session keeps a function_call that no function_response answers.

That history is replayed on every later turn, so the session does not recover on its own. A provider that requires strict pairing rejects the whole conversation: Anthropic answers tool_use ids were found without tool_result blocks immediately after, and the session stays unusable until it is deleted.

_drop_orphaned_function_responses already covers the mirror case, a response with no call. The unanswered call has no owner.

Change

Pair every function call with a result while the request contents are assembled, in _get_contents. A call the immediately following content does not answer gets a placeholder result there.

• The stored events are untouched, so recorded history stays intact and a session that is already broken heals on its next turn with no migration.
• It sits above the session service, so one implementation covers every store and every provider.
• Ids are read after the conversion, so a call whose adk- id was stripped is answered by a response with the same stripped id.
• Pairing is checked positionally, against the immediately following content, because that is the invariant the provider enforces.
• On a turn that is already partly answered, the placeholder joins the leading run of responses. Anthropic requires the results to precede any other block in the message, so appending after a trailing text part would be rejected for the same reason the missing result was.

A call the framework holds open (a long-running tool, an approval, a request for user input) is described as awaiting a response, not as having returned nothing. The difference changes what the model does next: told a tool returned nothing, it reissues the call or proceeds without it; told the call is still awaiting a response, it can wait. Those ids come from long_running_tool_ids, which lives on the event and does not survive the conversion to contents. Only a call with no recorded response is logged, so a call that is merely awaiting an answer stays quiet across the turns it spans.

A conversation whose calls are all answered is returned unchanged.

The mirror change for adk-go is google/adk-go#1405.

Testing plan

uv run pytest tests/unittests — 13067 passed, 2 pre-existing failures in test_import_loading.py that are present on main as well.

Seven new cases in tests/unittests/flows/llm_flows/test_contents_function.py: the interrupted turn at the end of the history and in the middle, the partially answered parallel turn, the same turn with a trailing text part, the call held open in both directions, and calls without an id. Two more convert the repaired contents through anthropic_llm.content_to_message_param and assert both halves of the provider invariant, that every tool_use block is answered by the next message and that the results precede any other block in it; a guard test asserts the same check rejects unrepaired input.

Two assertions in tests/unittests/apps/test_compaction.py are updated, because a pending call now reaches the model with a placeholder.