Qute: fix namespace build-time validation

[mkouba]

• when a matching template extension method exists
• this is a follow-up of Qute: add some more built-in string extensions #46258

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit 4579ca4.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.

[gsmet]

I added the backport label as I think it's necessary, remove it if not :)