[OpenAPI] Avoid caching request-specific spec URLs in HTML docs

[RedZapdos123]

Description:

Issue #353 reported that the cached OpenAPI HTML page embeds a request-specific spec URL from the first request.

This PR makes the cached HTML page request-independent by deriving the JSON spec URL from window.location.pathname in the browser instead of rendering rack.url_scheme, HTTP_HOST, and SCRIPT_NAME into the cached page.

It also adds a regression spec that covers repeated HTML requests with different hosts and mount paths, and adds a changelog entry for the fix.

Before fix:

• first request under /docs-a: http://first.test:3000/docs-a/json
• second request under /docs-b: http://first.test:3000/docs-a/json
• result: cached HTML reused the first request's spec URL.

After fix:

• cached HTML no longer embeds request host or mount path.
• Swagger UI resolves the JSON spec URL from the current window.location.pathname.
• the cached page works correctly across repeated requests with different hosts and mount paths.

Closes #353.

Checklist:

• I have run focused tests using bundle exec rspec spec/openapi/openapi_spec.rb.
• I have run broader OpenAPI tests using bundle exec rspec spec/openapi.
• I have run the full test suite using bundle exec rspec.