Add build validation pipeline status column to PR list

[tacoxse]

Summary

Addresses the build status part of #1 — adds a Build column to the pull request list (between Merge Status and Comments) showing the status of the build validation pipelines for each PR.

• Data source: the Policy Evaluations API (PolicyRestClient.getPolicyEvaluations), the same source the native PR page uses for build validation checks — filtered to enabled, non-deleted build policies, so both required and optional build validation are covered. No new extension scopes needed (vso.code suffices).
• Rendering: one aggregated status icon per PR — failed if any pipeline is rejected/broken, else running, else queued, else succeeded. The tooltip breaks down each pipeline by name and status. Shows … while loading, and an empty cell when no build validation policy applies.
• Fetching: mirrors the existing comment-threads pattern — lazy per-visible-PR, 6-way concurrency cap, batched setState, module-level cache shared across component instances, graceful fallback to empty on API errors.

Per CONTRIBUTING.md: README updated with the interface change, manifest version bumped 1.0.43 → 1.0.44.

Testing

• tsc --noEmit --skipLibCheck clean; production webpack build succeeds.
• Verified visually against a live Azure DevOps organization (private packaged build): aggregated icon and per-pipeline tooltip render correctly for PRs with build validation policies; PRs without applicable policies show an empty cell.

Notes

• The filter-by-build-status part of Show comments count, PR associated CI/Optional build status, and additional filters #1 is not included; it would layer naturally on top of this column as a follow-up.

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