ci: update d-e workflows for labeled on pull_request trigger

[grayside]

Description

Implementation of #3747, sibling of #3748.

Scoped to Samples workflows with file names starting with d-e, like: datastore, dialogflow, discoveryengine, document-ai, document-warehouse, endpoints, error-reporting, & eventarc.

This batch is by github workflow filename.

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

[grayside]

Test failure seems unrelated.

[subfuzion]

Test failure seems unrelated.

Concur, opened b/355236484 for following up separately.