Fixes #14238.

Root cause

Gateway delivery suppression treated response_previewed=True as proof that the final reply had reached the user. During compression/session handoff, unrelated interim commentary could be delivered while the actual final response still needed the normal platform send path. That left the response persisted in the child session but not sent to chat.

Fix

• Track exact commentary text delivered by GatewayStreamConsumer.
• Only suppress the normal final send when the stream consumer confirms final delivery, or when the exact final response text was already delivered as a preview.
• Apply the same guard before processing queued follow-up messages so the first response is not skipped at a session boundary.

Tests

• python -m pytest tests/gateway/test_run_progress_topics.py::test_run_agent_previewed_final_marks_already_sent tests/gateway/test_run_progress_topics.py::test_run_agent_previewed_split_keeps_final_delivery_pending tests/gateway/test_run_progress_topics.py::test_run_agent_queued_message_does_not_treat_commentary_as_final tests/gateway/test_run_progress_topics.py::test_run_agent_streaming_does_not_enable_completed_interim_commentary tests/gateway/test_stream_consumer.py::TestInterimCommentaryMessages -q --tb=short
• python -m pytest tests/gateway/test_run_progress_topics.py tests/gateway/test_stream_consumer.py -q --tb=short
• git diff --check

Note: the current full-suite baseline on main is covered by #13352, which is green and mergeable. If this PR shows the same unrelated full-suite failures before #13352 lands, the targeted regression coverage above is the relevant signal for this patch.