Add IBM Watson Text to Speech as an enterprise cloud provider option

Start by locating the provider abstraction and existing provider implementations, then inspect how settings, audio results, errors, mocked tests, and setup documentation are organized. The work is done when IBM Watson configuration and capability differences are handled consistently, responses are normalized, mocked tests pass, and setup documentation is updated.

IBM Watson Text to Speech is useful for enterprise-oriented workflows, custom/branded voices, and organizations already invested in IBM Cloud or Watson services. It should be considered after the provider abstraction and higher-priority providers are in place.

Impact:

• Enterprise users who rely on IBM Cloud cannot use their preferred TTS backend.
• The app has no path for Watson-specific voice customization or branded voice workflows.

Suggested fix:

• Add an IBM Watson TTS provider implementation behind the provider abstraction.
• Add settings for IBM API key, service URL/region, voice, output format, and any supported SSML controls.
• Normalize Watson responses into the same audio result model used by other providers.
• Surface provider-specific errors and unsupported capability differences clearly.
• Add mocked provider tests and setup documentation.