The canonical openai_codex SDK can complete a turn successfully
without emitting any `message.delta` events: the final assistant
text arrives only as an `ItemCompletedNotification` whose item is
an `agentMessage`. Before this change `_stream_thread_run` would
loop through the stream, see no delta text, return, and Studio
would emit only the empty usage + stop + `[DONE]` frames -- the
user sees a blank reply for what was actually a complete answer.
Track agent-message texts collected during the stream loop and, if
no streamed deltas came through, yield the last one before
returning. The buffered `thread.run()` fallback is still gated by
the existing `emitted_any` flag so it never replays a turn that
already executed side effects (file writes, shell commands).
`_completed_agent_message_text` accepts both the upstream object
shape (`ItemCompletedNotification(item.root.text=...)`) and the
dict shape pre-release builds and tests use, so it works across SDK
revs without an explicit version gate.
Tests: new regression
`test_empty_stream_falls_back_to_completed_agent_message` exercises
a fake SDK whose `turn().stream()` yields only an `item.completed`
event with an `agentMessage`; the test asserts the final text
reaches the visible chat output and that the buffered `run()` path
is NOT re-executed.
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