test(streaming): add coalescing-layer tests; loosen one model assertion

After merging the test-suite repair from main (#334) into this branch, one
model test (test_responses_api_streaming) regressed because its
assert_called_with strict-matched all kwargs of streaming_task_message_context
and didn't tolerate the new `streaming_mode='coalesced'` kwarg this PR
adds. Switched to assert_called() + targeted kwarg checks so the test
verifies what it cares about (task_id threading) without locking in
implementation details.

Replaced the ad-hoc smoke scripts that lived in conversation with a real
pytest module at tests/lib/core/services/adk/test_streaming.py covering:

- _delta_char_len, _can_merge, _merge_pair: per-channel correctness +
  None-handling
- _merge_consecutive: pure-text collapse, cross-channel order preservation,
  per-channel reconstruction matches per-token semantics
- CoalescingBuffer: first-delta-immediate flush within ~20ms,
  size-threshold flush before timer fires, multi-delta coalescing within
  one window, idle close, add-after-close no-op
- CoalescingBuffer cancel-during-flush regression test for the P1 fix:
  five queued chunks must all surface across publishes when close()
  cancels mid-flush (asserts substring presence rather than exact
  ordering, since the documented trade-off allows duplicates of the
  in-flight item)
- StreamingTaskMessageContext mode dispatch: "off" suppresses publishes
  but persists full content, "per_token" publishes each delta synchronously,
  "coalesced" batches and persists full content

Author: smoreinis

src/agentex/lib/core/temporal/plugins/openai_agents/tests/test_streaming_model.py

@@ -773,11 +773,13 @@ async def test_responses_api_streaming(self, streaming_model, mock_adk_streaming
             task_id=sample_task_id
         )
 
-        # Verify streaming context was created
-        mock_adk_streaming.streaming_task_message_context.assert_called_with(
-            task_id=sample_task_id,
-            initial_content=mock_adk_streaming.streaming_task_message_context.call_args.kwargs['initial_content']
-        )
+        # Verify streaming context was created with the right task_id. We
+        # don't strict-match the full kwargs because production also passes
+        # ``streaming_mode``, which is an implementation detail this test
+        # doesn't care about.
+        mock_adk_streaming.streaming_task_message_context.assert_called()
+        call_kwargs = mock_adk_streaming.streaming_task_message_context.call_args.kwargs
+        assert call_kwargs['task_id'] == sample_task_id
 
         # Verify result is returned as ModelResponse
         from agents import ModelResponse