fix: dedupe and report automatic template creation from images

install_appliances_from_image relied on the name+version pair check in
TemplatesService, so the same appliance reached through a second image
(the CSR1000v case) created a second template sharing the name. The auto
path now skips when any template with the same name exists, whatever the
version, and returns a manifest of created and skipped candidates;
POST /images/install replies 200 with that manifest instead of an empty
204, and the image_install MCP tool surfaces it.

gns3server/agent/mcp/__init__.py

@@ -1435,7 +1435,9 @@ async def image_install() -> list[dict[str, Any]]:
 
     This is NOT for downloading images. Images must be uploaded first (via the GNS3 Web UI).
     If an uploaded image matches a known appliance, a template is automatically created.
-    Images already referenced by existing templates are skipped.
+    Returns {"created": [...], "skipped": [...]}: images already referenced by existing
+    templates are skipped, and no template is auto-created when one with the same name
+    already exists (regardless of version).
     """
     return await asyncio.to_thread(_run_handler_sync, install_images_handler, {})