Add support for OpenAI text-to-speech instructions parameter

[faisal-alvi]

Description of the Change

This PR adds support for OpenAI's instructions parameter in the text-to-speech feature, allowing users to control voice characteristics using natural language instructions.

Changes made:

• Added a new "Voice instructions" textarea field to the OpenAI Text-to-Speech settings

• Updated default settings to include the instructions parameter

• Modified the API request to include instructions when provided

• Added get_instructions() method with filter support

• Added two new filters: classifai_openai_text_to_speech_instructions and classifai_openai_text_to_speech_request_body

• Users can now control voice characteristics with natural language (e.g., "Speak in a calm, professional tone")

• Provides more flexibility in audio generation

• Maintains backward compatibility (instructions are optional)

Approaches considered:

• Initially considered making this a universal setting across all TTS providers, but research showed that each provider uses different approaches (OpenAI uses natural language, Amazon Polly uses SSML, Azure uses SSML tags, ElevenLabs uses technical parameters)
• Decided to keep this OpenAI-specific to maintain clean separation of concerns

Closes #998, Closes #997

How to test the Change

1. Build the frontend assets:

   npm run build

2. Access the settings:

   • Go to WordPress Admin > Tools > ClassifAI > Language Processing > Text to Speech > Settings
   • Select "OpenAI Text to Speech" as provider
   • Verify the new "Voice instructions" field appears below "Audio speed"

3. Test the functionality:

   • Enter test instructions like "Say Welcome before starting", "Speak in a calm, professional tone" or "Use a more energetic delivery"
   • Save settings
   • Create/edit a post and generate text-to-speech audio
   • Verify the audio reflects the voice characteristics specified in instructions

4. Test filters:

   add_filter( 'classifai_openai_text_to_speech_instructions', function( $instructions ) {
       return $instructions . ' Speak with enthusiasm.';
   });

Changelog Entry

Added - Support for OpenAI text-to-speech instructions parameter to control voice characteristics

Credits

Props @swissky @dkotter @faisalalvi

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.