ci: remove use of pull_request_target (part 1)

[grayside]

Description

This change is the start on removing pull_request_target from sample testing workflows.

1. Templates are updated to add the labeled type to the pull_request trigger, and remove the pull_request_target.
2. The ai-platform-snippets.yaml workflow is updated to match
3. The reused remove-label.yaml workflow is converted to a no-op echo statement providing maintainer guidance
4. CONTRIBUTING.md is updated to note this behavior around PR testing controls

Workshop the pattern and guidance here, then migrate remaining workflows to this trigger configuration in follow-up.

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[iennae]

This only triggers a subset of tests right?

[grayside]

Yes, this is for the sample tests, as distinguished from all validations/checks that are run.

Added a comment with a suggestion to clarify.

[iennae]

minor doc updates/subtle log message change, but I'm cool with this as is as an incremental improvement towards better and getting us to testing this and validating this approach.

[grayside]

It appears there are multiple problems with the ai-platform-snippets, but fixing them is out of scope here. Going to swap them out for another test suite to make sure we have an end-to-end example of this approach working.

[grayside]

Added the actions:force-run label, and both the existing pull_request_target and the new pull_request triggers are activated. The former would be expected to remove the label, but it turns out most of the sample test workflows don't clean-up the label anyway.