feat: surface ambiguous merge insert error as InvalidInput

[wjones127]

Make ambiguous merge insert surfaced as InvalidInput and make the error message clearer that it aborted the write and not that data corruption might have already happened.

[github-actions]

Review

No P0 or P1 issues found. The changes are clean and correct.

handle_stream_processing_error — sending error to only one channel: This is safe because the two streams are consumed via .chain() (create_ordered_update_insert_stream, line 373), so an error yielded from update_stream will abort before insert_stream is ever polled. The fallback path (sending to insert_tx when update_tx's receiver is already dropped) correctly preserves the original error.

Nit (non-blocking): The two return; statements in handle_stream_processing_error are redundant since the function returns at the end of the match anyway.

LGTM.

[wkalt]

lgtm pending CI