Andrew Heard 384fed9a83 feat(ai): support speech metadata on text parts (#16749) ...

Add SpeechMetadata (public preview) so requests to Gemini
text-to-speech models can attach turn-level instructions to a
TextPart:

- speaker assigns the turn to a voice configured in
  MultiSpeakerVoiceConfig. Multi-speaker requests require it on
  every part.
- style describes sustained delivery, such as emotion, pacing, or
  volume, so it isn't read aloud as part of the transcript.
  Momentary sounds and pauses stay inline as vocal tags, such as
  <laugh> or <short pause>.

Set metadata with the new TextPart.init(_:speechMetadata:). The
existing init(_:) is kept so references like
[String].map(TextPart.init) still compile. TextPart.speechMetadata
and the SpeechMetadata properties are internal while the API is in
preview.

InternalPart encodes and decodes speechMetadata on the wire. History
doesn't merge streamed text parts that carry speech metadata, so
speaker turns stay separate.

Document the multi-speaker requirements on SpeechConfig,
MultiSpeakerVoiceConfig, and SpeakerVoiceConfig.

Replace the gemini-3.1-flash-tts-preview integration tests with a
TextToSpeechTests suite covering gemini-3.8-flash-tts and
gemini-3.8-flash-lite-tts in unary and streaming, single- and
multi-speaker requests.

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..

TestApp

feat(ai): support speech metadata on text parts (#16749)

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Unit

feat(ai): support speech metadata on text parts (#16749)

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