By default all internal HTTP requests are now blocked, unless mailpit is started with the `--allow-internal-http-requests` flag (env `MP_ALLOW_INTERNAL_HTTP_REQUESTS=true`).
This fix prevents unrestricted network probing via the screenshot proxy by limiting requests to images, fonts and CSS links found within a message, and returns a generic HTTP error to the client when unsupported content types are requested, not found, or otherwise disallowed.
See CWE-918 Server-Side Request Forgery (SSRF)
- Updated error handling to use the error return value for resource closures in tests and functions, ensuring proper error reporting.
- Replaced direct calls to `Close()` with deferred functions that handle errors gracefully.
- Improved readability by using `strings.ReplaceAll` instead of `strings.Replace` for string manipulation.
- Enhanced network connection handling by adding default cases for unsupported network types.
- Updated HTTP response handling to use the appropriate status codes and error messages.
- Removed unused variables and commented-out code to clean up the codebase.
