refactor(tts): migrate OpenAI TTS to shared TextToSpeechModel interface and standardize

[apappascs]

BREAKING CHANGE: OpenAI TTS now uses shared interfaces instead of provider-specific classes

• Replace deprecated OpenAI-specific classes (SpeechModel, SpeechPrompt, SpeechResponse, StreamingSpeechModel) with shared interfaces (TextToSpeechModel, TextToSpeechPrompt, TextToSpeechResponse, StreamingTextToSpeechModel)
• Update interface hierarchy: TextToSpeechModel now extends StreamingTextToSpeechModel for consistency with ChatModel pattern
• Add @FunctionalInterface annotation to StreamingTextToSpeechModel
• Migrate speed property from Float to Double across all OpenAI TTS components for consistency with ElevenLabs API and better type uniformity
• Update OpenAiAudioSpeechOptions to implement TextToSpeechOptions interface
• Add TextToSpeechModelTests with 7 comprehensive unit tests for interface behavior
• Improve null safety in TextToSpeechModel.call(String) default method
• Update documentation to reflect new interface hierarchy and Double usage
• Add migration guide to documentation for upgrading from deprecated classes

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[apappascs]

@ericbottard @markpollack
this commit is the follow up after the updates on OpenAI transcribe and TTS models and Implement ElevenLabs Text-to-Speech which added the shared interfaces and deprecate the audio speach classes that where placed inside the openai package as the only provider at the time.