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.

To run issue-labeler over these PRs as well as other PRs that have missed labeling (such as GitHub service interruptions when the events don't trigger), the PR label prediction workflow adds a cron schedule for as frequently as GitHub will run it (by using every 5 minutes). When triggered from the polling event, we collect the list of open and unlabeled PRs that have been updated since the last polling run (plus 5 extra minutes), and we run the issue-labeler prediction against those.

This also updates the workflow_dispatch event to allow an empty list of PR numbers to force a polling event run.

Changes

• Schedule trigger: Runs every 5 minutes (*/5 * * * *) to poll for unlabeled PRs
• poll-pull-requests job: Queries open PRs updated since last successful run, filters those without Area- labels
• workflow_dispatch enhancement: pulls parameter now optional; empty value triggers polling behavior
• predict-pull-label updates: Routes PR numbers based on trigger type (manual with PRs, polling, or automatic on open)
• Configuration: Added DEFAULT_LABEL: "needs-area-label", retained existing LABEL_PREFIX: "Area-"

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 from dotnet/.github#20 into this repository, updating the workflow for labeling pull requests. Retain the existing configuration for the default label and the label prefix while adding the functionality for polling and allowing manual poll triggers. The PR should be titled, "Run the issue-labeler over pull requests using polling" and it should have a description of: 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.

To run issue-labeler over these PRs as well as other PRs that have missed labeling (such as GitHub service interruptions when the events don't trigger), the PR label prediction workflow adds a cron schedule for as frequently as GitHub will run it (by using every 5 minutes). When triggered from the polling event, we collect the list of open and unlabeled PRs that have been updated since the last polling run (plus 5 extra minutes), and we run the issue-labeler prediction against those.

This also updates the workflow_dispatch event to allow an empty list of PR numbers to force a polling event run.

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.

Created from Copilot CLI via the copilot delegate command.

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