Declare the XML content type the Abri relay requires 🟩

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infrastructure/abri/abri_client.py

@@ -142,7 +142,11 @@ class AbriClient:
 
     def _post(self, envelope: bytes) -> bytes:
         try:
-            response = self._session.post(self._config.endpoint_url, data=envelope)
+            response = self._session.post(
+                self._config.endpoint_url,
+                data=envelope,
+                headers={"Content-Type": "application/xml"},
+            )
             response.raise_for_status()
         except requests.RequestException as error:
             raise AbriTransportError(str(error)) from error