Support `url_citation` annotations in `.outputTextAnnotationAdded` events

[richarddas]

Current behavior:

AIProxySwift 0.141.0 supports .outputTextAnnotationAdded for .fileCitation only. Web search citations aren't available as annotations.

What's missing:

The Responses API includes url_citation annotations with start_index/end_index indicating where URLs are cited in the text. These should be exposed via .outputTextAnnotationAdded, similar to .fileCitation.

Why it's needed:

Annotation indices allow precise detection of citation positions without regex parsing. This is more reliable and matches how file citations are handled.

Reference:

OpenAI docs:

1. response.output_text.annotation.added with url_citation type containing url, title, start_index, end_index.

2. https://platform.openai.com/docs/guides/tools-web-search