Set REQUEST_URI_HEADER on proxy client response before entity read

ModelProvider / HtmlJsonLDReader requires a base URI hint (set via
REQUEST_URI_HEADER on the response) to resolve relative references when
parsing HTML+JSON-LD. Without it jsoup throws a NullPointerException.
Mirrors the header injection done in the old ProxiedGraph.get().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: namedgraph

src/main/java/com/atomgraph/linkeddatahub/server/filter/request/ProxyRequestFilter.java

@@ -180,6 +180,8 @@ else if (agentContext instanceof IDTokenSecurityContext idTokenSecurityContext)
 
             try (clientResponse)
             {
+                // provide the target URI as a base URI hint so ModelProvider / HtmlJsonLDReader can resolve relative references
+                clientResponse.getHeaders().putSingle(com.atomgraph.core.io.ModelProvider.REQUEST_URI_HEADER, targetURI.toString());
                 requestContext.abortWith(getResponse(clientResponse));
             }
         }