fix(cli): align --url validation error message with test expectations

The Behave test expected the error message to contain '--url is only valid
for git resources' but the code was producing '--url only for git resources'.
Updated the validation error message to match the test assertion.

Also updated the Robot Framework integration test to expect the corrected
message, ensuring consistency across both test suites.

Fixes the failing Behave scenario: 'Using --url with a non-git resource type
is rejected' by correcting the error message text output from the CLI.

ISSUES CLOSED: #6322

robot/resource_cli.robot

@@ -53,4 +53,4 @@ Git Resource Add URL Flag Validation Rejects Non Git Types
     ...    timeout=120s    on_timeout=kill    stderr=STDOUT
     Log    ${result.stdout}
     Should Not Be Equal As Integers    ${result.rc}    0
-    Should Contain    ${result.stdout}    --url only for git resources
+    Should Contain    ${result.stdout}    --url is only valid for git resources

src/cleveragents/cli/commands/resource.py

@@ -699,7 +699,8 @@ def resource_add(
         if url is not None:
             if type_name != "git":
                 console.print(
-                    "[red]Validation error:[/red] --url only for git resources."
+                    "[red]Validation error:[/red] "
+                    "--url is only valid for git resources."
                 )
                 raise typer.Abort()
             url_value = url.strip()