Route vendored-sync drift issues to area triage via manifest-declared labels

[Evangelink]

Follow-up to the vendored-files drift detector, prompted by @marcpopMSFT's question on #55471: drift issues only carried area-vendored-sync, so they never reached an area triage queue.

Every entry in eng/vendored-files.json is dotnet test ↔ Microsoft.Testing.Platform shared source, so those issues now also get Area-dotnet test (MTP).

The detector itself is upstream-agnostic — each source declares its own repo/ref/path, so it can track a file vendored from any repository. Hard-coding the MTP label in the script would therefore silently misroute a future entry vendored for a different area. Area routing is manifest data instead:

• eng/vendored-files.json gains a manifest-level default_area_labels, plus an optional per-entry area_labels override.
• Entry.issue_labels resolves the label set per entry; issue creation, label backfill, and label creation all go through it.
• Existing open issues ([vendored-sync] dotnet-test-wire-contract-constants: drift detected (#0) #55471–[vendored-sync] dotnet-test-terminal-ansiterminaltestprogressframe: drift detected (#0) #55474) are backfilled with any missing label on the next scheduled run, so no manual relabelling is needed.
• ensure_label still force-creates the workflow-owned sync label, but only creates area labels when missing — the repo's triage taxonomy owns their description/colour, and the workflow must not overwrite it.
• validate rejects empty/non-string area_labels and re-declaring the sync label.

Docs updated: manifest schema, an explicit note that a differently-scoped entry must set area_labels, and a step in Adding a new vendored file.

Testing

• check_vendored_files.py validate passes (30 entries, 38 sources).
• Unit-checked label resolution: manifest default, per-entry override, explicit-empty override (sync label only), no list aliasing between entries, and validate rejecting malformed area_labels.
• Live check --dry-run against microsoft/testfx: 4 drifted, 0 errors, each resolving area-vendored-sync, Area-dotnet test (MTP).

No product code is touched — this only affects the labels on issues the weekly workflow opens.

[azure-pipelines]

Azure Pipelines:
Successfully started running 2 pipeline(s).
1 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR updates the vendored-file drift detector workflow so drift issues are routed into the correct area triage queues by applying area labels that are declared in the eng/vendored-files.json manifest (with a manifest-level default and per-entry overrides), rather than hard-coding labels in the script.

Changes:

• Add default_area_labels at the manifest level, with optional per-entry area_labels overrides.
• Update the drift detector script to resolve per-entry label sets, apply them on issue create, and backfill missing labels on existing open drift issues.
• Update documentation and workflow comments to describe the new label-routing behavior.

Show a summary per file

File	Description
eng/vendored-files.md	Document manifest schema changes and clarify how/when to set area_labels.
eng/vendored-files.json	Add default_area_labels for the current set of dotnet test/MTP vendored entries.
.github/workflows/check-vendored-files.yml	Update workflow commentary to note issues include area labels from the manifest.
.github/scripts/check_vendored_files.py	Resolve/apply area labels per entry, backfill missing labels on existing issues, and adjust label creation behavior.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Evangelink]

@marcpopMSFT could you please review?

[marcpopMSFT]

@marcpopMSFT could you please review?

I'm not really familiar with what you're doing here so not sure how much help I can be. I had copilot do a local review and asked a few questions and it's going to add those comments to the review.