Run the issue-labeler over pull requests using polling

[Copilot]

Copilot-authored pull requests run workflows with permissions lower than other PRs (including those submitted by external contributors). This blocks the issue-labeler workflow from being able to run on those PRs, leaving a prompt for the workflow to be approved.

Changes

• Add cron schedule - Poll every 5 minutes for open, unlabeled PRs that have been updated since the last polling run (plus 5 minute overlap)
• Make workflow_dispatch input optional - Allow empty PR number list to force polling behavior
• Add poll-pull-requests job - Query open PRs, filter to those missing area-* labels, output comma-separated list
• Update predict-pull-label job - Add step to determine PR source (workflow_dispatch input, polling output, or event number), skip processing when list is empty

This catches PRs that miss labeling due to permission restrictions or GitHub service interruptions.

Addresses (dotnet/issue-labeler#105) Pull Request Labeling does not run automatically for Copilot PRs for this repository.

Replicates dotnet/.github#20 and dotnet/runtime#124023.

Original prompt

Replicate the change in dotnet/runtime#124023. Use the same PR title and description, adding a link to the dotnet/runtime#124023 PR in the bottom of the description for reference. Do not change any of the other issue labeling configuration.

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[Copilot]

Pull request overview

Updates the pull-request label prediction workflow to support a polling mode (via cron schedule or empty workflow_dispatch input) so PRs that missed labeling due to reduced token permissions (e.g., Copilot-authored PRs) can still get area-* labels.

Changes:

• Adds a 5-minute cron schedule to poll for open PRs missing area-* labels.
• Makes the workflow_dispatch pulls input optional, where an empty value triggers polling behavior.
• Introduces a poll-pull-requests job and updates the prediction job to determine which PR numbers to process (and skip work when none are found).

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