Changes

Part of #2030 (see the scope notes below)

The GitLab interceptor skips token verification when secretRef is not set and accepts every request without any indication. As proposed in the issue, this logs a warning on each request processed without a secretRef, and the docs for the GitLab and Bitbucket server interceptors now say that omitting secretRef disables verification.

Notes on scope:

• The GitHub interceptor already requires secretRef, so it needs no change.
• I did not add the warning to the Bitbucket interceptor: Bitbucket Cloud does not support shared secrets at all, so every cloud user would get a warning they cannot act on. Happy to add it for server-style requests if there is a reliable way to tell them apart.
• The admission webhook warning from the issue (item 2) is not part of this PR.

Tests: TestInterceptor_Process_WarnsWithoutSecretRef checks that the warning is logged without a secretRef and not with one. go test ./pkg/interceptors/gitlab/ passes.

/kind feature

Submitter Checklist

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Tested your changes locally (unit tests only)
• Follows the commit message standard
• Meets the Tekton contributor standards
• Has a kind label

Release Notes

The GitLab interceptor logs a warning when it processes a request without a secretRef, because the webhook token is then not verified.