Add anthropic-beta header for structured outputs.

[johnhooks]

When requesting structured JSON output from the Anthropic API using the output_format parameter with a JSON schema, the API requires the anthropic-beta: structured-outputs-2025-11-13 header to be present in the request. Without this header, Anthropic rejects the request because structured outputs is currently a beta feature that must be explicitly opted into. This fix detects when JSON schema output is configured via getOutputMimeType() and getOutputSchema() and automatically adds the required beta header, allowing structured output requests to succeed.

See Claude Docs: Structured outputs

Fixes #151.

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[johnhooks]

@JasonTheAdams I was going to add a test or two, but at the moment there aren't any tests for AnthropicTextGenerationModel, or am I missing something?

[JasonTheAdams]

Thanks, @johnhooks! You're right. There aren't currently any tests. I'll let @felixarntz decide if he'd like to add tests in this PR since he built out #156. I'm also adding him as a reviewer.

[johnhooks]

I had tests in my implementation, before #156 was created AnthropicTextGenerationModelTest.php. I could attempt to port it over and add tests for all of AnthropicTextGenerationModel.

UPDATE: I updated the test for the current implementation, and intended to stack the PRs, but I don't think I can since the incoming branch is from my fork. But I can either do separate PR, or merge into this one. https://github.com/johnhooks/php-ai-client/blob/cbaf7dc4f634bc7e6bd8676a393d600eb05034bc/tests/unit/ProviderImplementations/Anthropic/AnthropicTextGenerationModelTest.php

[felixarntz]

@johnhooks I think this is good to go. While we should probably have tests for these, I think this is its own task to do, and this change is simple enough to not require being blocked to add tests.

Great catch, thank you for fixing this!