Sibling of the chat_completions zero-byte-args fix (previous commits):
a clean SSE close after content_block_start(tool_use) but before any
input_json_delta / message_delta yields an SDK final-message snapshot
whose content is NON-empty (the tool_use block is present, input={})
and whose stop_reason is None. That shape sailed past both
empty-stream guards (they only fire on empty content) and executed the
tool with empty input — no retry, no error: the same silent-data-loss
class as #80498, one provider transport over.
A legitimate completion always carries a stop_reason, so a
tool_use-bearing message without one is a mid-tool-call stream drop.
Raise EmptyStreamError for it, riding the same bounded stream-retry
(HERMES_STREAM_RETRIES) the eventless-stream case already uses.
Gate checked on both return paths (raw SDK snapshot and
accumulator-modified message). Regression tests cover the dropped
shape (mutation-verified: disabling the gate fails exactly that test),
the legitimate tool_use completion, and the text-only no-stop_reason
shape (pre-existing behavior preserved).
kshitij