Summary

Fixes #1628.

When using ClientSpecifiesVersionInHeader, responses can differ based on the version request header, but the server wasn't setting the Vary response header accordingly. Caching proxies could then serve a response cached for one API version to a client requesting another.

This adds DynamicVersionPolicy::response_vary_on() (default: none) so policies can declare which request headers affect the response. ClientSpecifiesVersionInHeader returns its configured header name. VersionPolicy::add_vary_headers() appends those fields to Vary without duplicating existing entries and skips when Vary: * is already set. The server applies this to all responses (including version-policy errors) in http_request_handle_wrap, matching the existing gzip Accept-Encoding pattern.

Testing

• Unit tests for add_vary_on_header (basic add, no duplicate, skip on Vary: *)
• test_versions integration test asserts Vary includes the version header on success and error responses
• cargo test -p dropshot