Use native request remote address for XForwarded trusted proxies

[lly835]

Fixes #4074.

Summary

• In XForwardedHeadersFilter, resolve remote address from ServerHttpRequestDecorator.getNativeRequest(request) instead of request.getRemoteAddress().
• Use that native peer address consistently for:
  • trusted-proxies check
  • X-Forwarded-For value appended by the filter
• Add a regression test (trustedProxiesUsesNativeRequestRemoteAddress) that decorates the request with a rewritten remote address and verifies trusted-proxies logic still uses the native connection peer.

Testing

• Attempted: ./mvnw -pl spring-cloud-gateway-server-webflux -Dtest=XForwardedHeadersFilterTests test
• Could not run in this environment because no Java runtime is installed (Unable to locate a Java Runtime).