Added details on request-variable substitution, explaining how it
reads fields from the active outbound request and resolves upstream
variables from the current route context.
Refactor response body modification to only allow text-like content types
(JSON, YAML, XML, etc.) instead of all HTML responses.
Body modification is now
blocked for binary content and transfer/content encoded responses, while status
code and headers can still be modified.
This prevents issues with compressed or streaming responses while
maintaining the ability to modify text-based API responses.