Add support for text to speech conversion

[dkotter]

What?

Adds support for text to speech conversion

Why?

We currently have a TODO statement around adding support for text to speech so this PR implements that. This allows others to build out text to speech systems using this provider plugin.

How?

• Introduce a new OpenAiTextToSpeechConversionModel that handles all requests to convert text to speech
• Ensure this model is loaded when a TTS generation request is made
• When determining which model capabilities we want for TTS, ensure we support the inline output file type

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Opus 4.8
Used for: Putting together a plan and executing on that plan. Plan reviewed and modified by me and all code was reviewed and tested by me

Testing Instructions

Hard to test on it's own as this plugin provides functionality but doesn't actually do anything with that. Easiest approach is the following:

1. Checkout the WordPress AI plugin from this PR
2. Download this PR, activate and configure the OpenAI Provider
3. In the AI settings page, turn on Text to Speech
4. Go to a post and find the Text to Speech panel in the sidebar
5. Click on the Generate Audio button and ensure it works as expected

Changelog Entry

Added - Support for text to speech conversion

[github-actions]

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Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: superdav42 <superdav42@git.wordpress.org>

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[superdav42]

Requesting one change before merge:

• src/Models/OpenAiTextToSpeechConversionModel.php:213: an empty successful response is malformed provider output, but this branch throws Common\Exception\InvalidArgumentException, which represents bad caller input. The SDK reserves Providers\Http\Exception\ResponseException for unexpected or malformed provider responses, and the existing response parsers in this repository use ResponseException::fromMissingData(). The current exception type can make consumers misclassify the failure and skip their provider-error handling or retry path. Please throw a ResponseException here (for example, for a missing response body) and add a focused regression test that passes an empty 2xx body.

Verification: reviewed head 954aa2e; PHP syntax and PHPCS pass for all three changed files against locked wordpress/php-ai-client 1.3.1.

[dkotter]

Requesting one change before merge:

• src/Models/OpenAiTextToSpeechConversionModel.php:213: an empty successful response is malformed provider output, but this branch throws Common\Exception\InvalidArgumentException, which represents bad caller input. The SDK reserves Providers\Http\Exception\ResponseException for unexpected or malformed provider responses, and the existing response parsers in this repository use ResponseException::fromMissingData(). The current exception type can make consumers misclassify the failure and skip their provider-error handling or retry path. Please throw a ResponseException here (for example, for a missing response body)

Good idea, updated in e5ac5a3

and add a focused regression test that passes an empty 2xx body.

Right now this plugin has no tests which is why I didn't add any as part of this PR. It seems like a separate PR would be better to get test harness in place but let me know if you'd like all of this in this PR instead