* fix(stream): don't report dropped mid-tool-call streams as output truncation
A streaming tool call whose SSE ends with no finish_reason (the upstream
delivers the tool name + opening '{' then closes the connection cleanly,
no terminator, no [DONE]) was stamped finish_reason='length' by the mock
builder. That routed it through the output-cap truncation path: 3 useless
max_tokens-boosted retries, then the misleading 'Response truncated due to
output length limit' error — even though the model never reported hitting
any cap.
Reproduced live on nvidia/nemotron-3-ultra:free via the Nous dedicated
endpoint, which stalls/drops during large tool-arg generation (50s-4m41s).
Now: when tool args are incomplete AND the provider sent no finish_reason,
tag the response as a partial-stream stub so the loop reports an honest
mid-tool-call drop and asks the model to chunk its output (existing
continuation machinery), instead of escalating output budget and lying.
A provider-reported finish_reason='length' still takes the real-truncation
path unchanged.
* test(stream): update truncated-tool-args test for drop-vs-cap split
test_truncated_tool_call_args_upgrade_finish_reason_to_length pinned the
old behaviour where ANY incomplete tool args → finish_reason='length' with
tool_calls preserved. That single-chunk-no-finish_reason scenario is exactly
the mid-tool-call stream drop now reclassified as a partial-stream stub.
Split into two tests matching the new contract:
- no finish_reason + incomplete args → PARTIAL_STREAM_STUB_ID, tool_calls=None,
_dropped_tool_names set (the drop path)
- explicit finish_reason='length' + incomplete args → tool_calls preserved,
'length' upgrade unchanged (the genuine output-cap path)
