chat_completions(stream=True, stream_metadata=True) used stream_metadata
only to pick the raw dict-chunk parser locally, never adding it to the
request payload. The wire request didn't match the caller's intent and
relied on the server sending metadata chunks unconditionally. Forward the
flag (mirroring the modern CloudBackend, which always sends it) so the
request is correct and robust if the server ever gates metadata behind it.
Verified against the real API that the server currently emits block_metadata
regardless, so this is a latent-correctness fix, not a behavior change today.
4c7d1088ba