NIFI-15953 Improve Proxy Host Header Documentation and Validation

[exceptionfactory]

Summary

NIFI-15953 Updates the Administrator's Guide documentation for the nifi.web.proxy.host header and implements additional validation for proxy host header values based on the configured property.

The documentation clarifies the behavior of Host header validation when running with HTTPS enabled, describing TLS Server Name Indication and DNS Subject Alternative Name requirements for server certificates. Property description details also describe expected behavior for reverse proxy servers that provide the X-ProxyHost or X-Forwarded-Host header values.

The additional Proxy Header Validator follows the same pattern as the Host Port Validator, returning an HTTP 421 Misdirected Request when receiving an invalid host header string.

The implementation and documentation maintain existing requirements that enforce validation when the application is running with HTTPS enabled. Unit test updates exercise the configured Proxy Header Validator and assert expected HTTP response status codes.

Tracking

Please complete the following tracking steps prior to pull request creation.

Issue Tracking

• Apache NiFi Jira issue created

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000
• Pull request contains commits signed with a registered key indicating Verified status

Pull Request Formatting

• Pull Request based on current revision of the main branch
• Pull Request refers to a feature branch with one commit containing changes

Verification

Please indicate the verification steps performed prior to pull request creation.

Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • JDK 25

Licensing

• New dependencies are compatible with the Apache License 2.0 according to the License Policy
• New dependencies are documented in applicable LICENSE and NOTICE files

Documentation

• Documentation formatting appears as expected in rendered files