docs: add streaming speaker diarization parameters to API reference

[devin-ai-integration]

Summary

Adds streaming speaker diarization parameters to both the Universal-Streaming and Universal-3 Pro Streaming API reference specs, based on the Streaming Diarization docs.

Connection parameters added (to both usm-streaming.yml and usm-streaming-u3-pro.yml):

• speaker_labels — boolean ("true"/"false", default "false") to enable real-time speaker diarization
• max_speakers — integer (1–10) to hint the expected number of speakers

Response field added (to TurnPayload schema in both specs):

• speaker_label — string (e.g. "A", "B", "UNKNOWN") on Turn events when diarization is enabled

Fern overrides updated to mark both new connection parameters as optional.

Review & Testing Checklist for Human

• Verify speaker_labels and max_speakers parameter names, types, defaults, and constraints match the actual streaming API behavior
• Confirm speaker_label should not be in the required array of TurnPayload (it's only present when speaker_labels is enabled)
• Check whether enable_extra_session_information or multichannel parameters (also on the docs page) should be added in this PR or a follow-up
• Build the Fern docs locally (pnpm run docs) and verify the new parameters render correctly on both streaming API reference pages

Notes

• The speaker_labels param uses enum: ["true", "false"] to match the convention of other boolean query params in these specs (e.g. language_detection, format_turns).
• The docs page also documents enable_extra_session_information and multichannel parameters, but they are not strictly diarization parameters — excluded based on the request scope.

Link to Devin session: https://app.devin.ai/sessions/fd93e5c0f422444a90a899b7734b9607
Requested by: @LeeVaughn

[devin-ai-integration]

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