fix: migrate OpenAI TTS provider to SDK v1.x

[kesslerio]

Problem

The OpenAI TTS provider fails with a 404 error when using OpenAI SDK v1.x:

Error code: 404 - Invalid URL (POST /v1/audio/speech)

This occurs because the provider uses the deprecated global API pattern from SDK v0.x:

• Setting openai.api_key globally
• Calling openai.audio.speech.create() at module level

OpenAI SDK v1.x requires instantiating a client object.

Solution

Migrate to the modern client-based pattern:

# Before (SDK v0.x - broken with v1.x)
openai.api_key = api_key
response = openai.audio.speech.create(...)

# After (SDK v1.x - compatible)
self.client = openai.OpenAI(api_key=api_key)
response = self.client.audio.speech.create(...)

Changes

• Initialize OpenAI() client in __init__ instead of setting global API key
• Use self.client.audio.speech.create() instead of module-level function
• Rely on client's automatic environment variable handling for auth

Testing

Tested with OpenAI SDK 1.109.1:

• TTS generation works correctly
• 404 error resolved

Compatibility

• Requires OpenAI SDK >=1.0.0 (which the package already depends on)
• No breaking changes to provider interface

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Note: This fix is urgent as the current code is broken for all users with modern OpenAI SDK versions.