feat(speechmatics): add max_speakers parameter for speaker diarization

[nsepehr]

Summary

This PR adds support for the max_speakers parameter to the Speechmatics STT plugin, allowing developers to limit the number of unique speakers detected during diarization.

Problem

Currently, when using the Speechmatics STT plugin with diarization enabled, there's no way to specify the maximum number of speakers. The transcription_config parameter (which is deprecated) accepts a speaker_diarization_config with max_speakers, but this value is not preserved when the plugin processes the configuration.

Solution

• Added max_speakers as a direct parameter to the STT __init__ method
• Updated the STTOptions dataclass to include the max_speakers field
• Modified _process_config to include max_speakers in the speaker_diarization_config when sending to the Speechmatics API
• Added proper handling for extracting max_speakers from the deprecated transcription_config parameter for backward compatibility
• Updated documentation to explain the new parameter

Use Case

This parameter is particularly useful for scenarios where the number of participants is known in advance, such as:

• Two-person interviews or conversations
• Small group discussions with a fixed number of participants
• Customer service calls (agent and customer)
• Educational settings with known speaker counts

Testing

• Tested locally with a multi-speaker agent implementation
• Verified that the parameter is correctly passed to the Speechmatics API configuration
• Confirmed backward compatibility with the deprecated transcription_config parameter

Example Usage

stt = speechmatics.STT(
    language="en",
    enable_diarization=True,
    max_speakers=2,  # Limit to 2 speakers
    diarization_sensitivity=0.5,
    speaker_active_format="@[{speaker_id}]: {text}",
)

Breaking Changes

None - this is a backward-compatible addition.

[CLAassistant]

All committers have signed the CLA.

[longcw]

speaker_diarization_config is a dataclass? if it requires a specific version of speechmatics, we can specify it in pyproject.toml.

Suggested change

	if not is_given(max_speakers) and hasattr(config, "speaker_diarization_config"):
	if (
	config.speaker_diarization_config
	and "max_speakers" in config.speaker_diarization_config
	):
	max_speakers = config.speaker_diarization_config["max_speakers"]
	if (
	not is_given(max_speakers)
	and (dz_cfg := config.speaker_diarization_config)
	and dz_cfg.max_speakers is not None
	):
	max_speakers = dz_cfg.max_speakers

[longcw]

we should also refactor _process_config to replace the dataclass value of transcription_config instead of assigning a dict to it.