fix: return the created template from appliance install

POST /appliances/{id}/install replied 204 with an empty body, so the MCP
appliance_install tool crashed with 'Expecting value: line 1 column 1'
while the template had actually been created. The route now returns the
created template (201, response_model=schemas.Template), _create_template
propagates it, and the MCP handler parses the body defensively so an
empty reply degrades to a plain success message.

gns3server/agent/mcp/__init__.py

@@ -1376,7 +1376,7 @@ async def appliance_install(
     appliance_id: Annotated[str, Field(description="UUID of the appliance to install")],
     version: Annotated[str | None, Field(description="Version to install (e.g. '2.7.0.356'). Required if the appliance has multiple versions. Use appliance_get to see available versions.")] = None,
 ) -> list[dict[str, Any]]:
-    """Create a template from a GNS3 appliance definition.
+    """Create a template from a GNS3 appliance definition and return the created template.
 
     NOTE: This does NOT download images. Images must be placed in the
     GNS3 images directory (e.g. ~/GNS3/images/) beforehand.

gns3server/agent/mcp/appliances.py

@@ -85,5 +85,13 @@ def install_appliance_handler(params: dict[str, Any], gns3_ctx: dict[str, Any])
     version = params.get("version")
     if version:
         request_params["version"] = version
-    result = conn.http_call("post", url, params=request_params).json()
-    return {"message": f"Appliance {appliance_id} installation requested", "result": result}
+    response = conn.http_call("post", url, params=request_params)
+    result = {"message": f"Appliance {appliance_id} installed"}
+    if response.content:
+        # the install endpoint returns the created template (201); tolerate an
+        # empty body in case an older server still replies with 204
+        template = response.json()
+        result["template"] = {
+            k: template[k] for k in ("template_id", "name", "version", "template_type") if k in template
+        }
+    return result