webhooks: Add nocoverage comment for unused MAPPING.

This code path isn't used yet; I expect we'll get to remove this when
we start using it for a specific integration in the near-ish future.

zerver/lib/webhooks/common.py

@@ -97,7 +97,7 @@ class WebhookUrlOption:
                     label="Exclude notifications from private repositories",
                     validator=check_bool,
                 )
-            case PresetUrlOption.MAPPING:
+            case PresetUrlOption.MAPPING:  # nocoverage # Not used yet
                 return cls(
                     name=config.value,
                     label="",