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Updated: Added tests and clarified filtering rationale

What this fixes

Issue #4261: When a GroupChat orchestrator terminates, it yields
self._full_conversation (the entire conversation history) via
ctx.yield_output(). Without filtering, WorkflowAgent converts all
messages — including user inputs and earlier assistant responses — into
output, causing them to compound across successive turns.

Root cause analysis

The termination path in BaseGroupChatOrchestrator:

# _check_terminate_and_yield / _check_agent_terminate_and_yield
await ctx.yield_output(self._full_conversation)  # list[Message]

This hits the list[Message] branch in both
_convert_workflow_event_to_agent_response_updates (streaming) and
_convert_workflow_events_to_agent_response (non-streaming), where all
messages were forwarded without filtering.

Fix: _filter_messages

Returns only the last meaningful assistant message from list[Message]
output. This is intentionally more aggressive than just filtering
role="user" because:

1. User messages — re-emitting these as assistant output is the
   reported bug
2. Earlier assistant messages — these are conversation history replay
   from _full_conversation. They were already surfaced in real-time
   during workflow execution via AgentResponseUpdate streaming. Including
   them again in the termination output causes duplication.
3. Default behavior alignment — GroupChatBuilder defaults to
   intermediate_outputs=False, meaning only the orchestrator's output
   is surfaced. Users who need intermediate agent responses can opt in
   via intermediate_outputs=True.

Relationship to #4275

PR #4275 filters the AgentResponse branch. This PR filters the
list[Message] branch, which is the actual code path hit by GroupChat's
termination yield (ctx.yield_output(self._full_conversation)). Both
fixes are complementary — they cover different branches of the same
conversion logic.

Tests added

• Updated test_workflow_as_agent_yield_output_with_list_of_chat_messages
  to reflect the new filtering semantics (last assistant message only)

• Added TestWorkflowAgentUserInputFilteringRegression with two
  multi-turn compounding regression tests:

  • test_streaming_compounding_not_observed_across_turns
  • test_nonstreaming_compounding_not_observed_across_turns

  These reproduce the exact escalating symptom from the issue report
  where turn 1's "hi" bled into turn 2's response.

@copilot review