fix(stream-consumer): preserve accumulated text on chunk send failure

[chinadbo]

Summary

• When _send_new_chunk fails during the chunk-splitting path (network error, send returns success=False), _accumulated was cleared unconditionally at line 193, causing silent text loss
• The fix tracks how many bytes of _accumulated were successfully delivered and only trims that portion; unsent text is preserved for retry
• Consistent with the existing _message_id is not None overflow path (lines 207-226) which already checks _send_or_edit return before advancing _accumulated

Test plan

• test_chunk_send_failure_preserves_accumulated — all chunk sends fail → _accumulated retains full text
• test_partial_chunk_failure_preserves_unsent_text — first chunk succeeds, second fails → unsent portion preserved
• All 28 existing test_stream_consumer.py tests still pass

[teknium1]

Thanks for tracing the overflow failure path. The current-head defect is real: gateway/stream_consumer.py:668-676 clears the buffer after chunk attempts even when a send fails, and a non-final iteration continues at line 692.

Problems

• gateway/stream_consumer.py:354 counts len(chunk) as source progress. BasePlatformAdapter.truncate_message() appends (i/N) to multi-message chunks (gateway/platforms/base.py:5620-5625), so this can over-trim _accumulated after a successful chunk.
• gateway/stream_consumer.py:357 retains the suffix while leaving _message_id on the successful chunk. A later non-final flush follows the edit path at current gateway/stream_consumer.py:694-748, which can replace that delivered chunk with the suffix instead of sending a continuation.

Suggested changes

• Track consumed source boundaries independently of decorated transport chunks, and introduce a continuation state for partial initial-overflow delivery.
• Add a regression where failure occurs before _DONE, later deltas arrive, and the final output contains the full text exactly once.

Automated hermes-sweeper review.

[teknium1]

truncate_message() decorates multi-message chunks with (i/N) (gateway/platforms/base.py:5620-5625), so len(chunk) is not the number of source characters consumed. This slice can skip source text after a successful decorated chunk; track raw source boundaries instead.

[teknium1]

After a partial success, _message_id still identifies the delivered chunk. A later non-final flush will enter _send_or_edit and edit that message with this retained suffix, replacing visible content rather than creating a continuation. The partial-failure state needs an explicit continuation transition.