AXIS2-6101 Implement getResponseContentEncoding() in H2 transports

H2RequestImpl and H2StreamingRequestImpl implement the new
Request.getResponseContentEncoding() method added for the HC5 5.6+
double-decompression fix. Both return null since the HTTP/2 async
pipeline handles decompression before the response body is exposed.

Fixes Java 17 CI build failure — these classes implement the
Request interface and must provide the new abstract method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: robertlazarski

modules/transport-h2/src/main/java/org/apache/axis2/transport/h2/impl/httpclient5/H2RequestImpl.java

@@ -224,6 +224,14 @@ public InputStream getResponseContent() throws IOException {
         return null;
     }
 
+    @Override
+    public String getResponseContentEncoding() {
+        // HTTP/2 async responses use SimpleHttpResponse which does not expose
+        // entity content encoding separately. Return null (no encoding) since
+        // HC5's async pipeline handles decompression before we see the body.
+        return null;
+    }
+
     @Override
     public int getStatusCode() {
         return response != null ? response.getCode() : -1;

modules/transport-h2/src/main/java/org/apache/axis2/transport/h2/impl/httpclient5/H2StreamingRequestImpl.java

@@ -278,6 +278,14 @@ public InputStream getResponseContent() throws IOException {
         return null;
     }
 
+    @Override
+    public String getResponseContentEncoding() {
+        // HTTP/2 streaming responses use SimpleHttpResponse which does not
+        // expose entity content encoding separately. Return null since
+        // HC5's async pipeline handles decompression before we see the body.
+        return null;
+    }
+
     @Override
     public void releaseConnection() {
         log.trace("Releasing HTTP/2 streaming connection resources");