test: improve DNS-1123 validation error assertion

Update the watchNamespace DNS-1123 validation test to assert on a more
meaningful portion of the error message. Instead of just checking for
the invalid value "invalid-namespace-", the test now verifies that the
error message includes "lowercase RFC 1123", which better demonstrates
that proper namespace name validation is occurring.

The error message from Kubernetes DNS-1123 validation includes detailed
information about RFC 1123 requirements, making this assertion more
robust and informative.

Assisted-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: camilamacedo86

test/e2e/features/install.feature

@@ -325,7 +325,7 @@ Feature: Install ClusterExtension
       """
     Then ClusterExtension reports Progressing as True with Reason Retrying and Message includes:
       """
-      invalid-namespace-
+      lowercase RFC 1123
       """
 
   @SingleOwnNamespaceInstallSupport