OpenAIFilterer ignores URL config, always hits api.openai.com

[joryirving]

The OpenAIFilterer in filter_openai.go does not have a URL field, so the URL configured in YAML is ignored. It always defaults to api.openai.com.

Compare with annotator_openai.go which correctly has:

• URL string field
• option.WithBaseURL(baseURL) when creating the client

The OpenAIFilterer struct is missing both.

Steps to Reproduce

1. Configure OpenAI filter with custom URL in YAML
2. Messages are processed but request goes to https://api.openai.com/v1/chat/completions
3. Results in 401 Unauthorized (no OpenAI API key)

Expected Behavior

The OpenAIFilterer should respect the URL config field and send requests to the configured endpoint.

Fix

Add URL field to OpenAIFilterer struct and use option.WithBaseURL() when creating the client, similar to how OpenAIAnnotator does it.