chore: Require acctest label for prs

[stevehipwell]

Resolves #ISSUE_NUMBER

This PR stops unnecessary acceptance test runs using up the API limit.

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Before the change?

• All non-fork PRs trigger acceptance tests

After the change?

• Non-fork PRs require the acctest label to be set

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[Copilot]

Pull request overview

These provider review instructions are being used. This PR adjusts the Acceptance Tests GitHub Actions workflow to avoid running acceptance tests on every same-repo PR update by requiring an explicit acctest label before tests will run in PR contexts, reducing unnecessary GitHub API usage.

Changes:

• Gate acceptance test execution for pull_request_target events on whether the acctest label is present.
• Simplify the PR fork vs. base-repo branching so the label gate happens once up-front.