[Bug]: Custom headers casing is changed?

### What happened?

I'm trying to use the inspector to test some custom extensions, so I'm setting the `X-A2A-Extensions` header and, somehow, the casing of the header name changes when the call is made.

### Relevant log output

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look at the inspector's request handling code, likely in a file that processes HTTP headers. The issue mentions a custom header 'X-A2A-Extensions' whose casing is being altered. Start by searching for header normalization or transformation logic. Check how headers are set in the UI and passed to the underlying HTTP client. Verify the behavior by running the inspector locally and inspecting network traffic.

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