CI: Nightly matrix failure reporter (artifacts + workflow_run aggregator)

[jbrinkman]

This PR implements the nightly matrix failure reporter described in .vs/error-workflow.md and addresses GH issue #71.

Closes #71

What’s included

• tests.yml: Ensure each matrix leg emits TRX and a normalized failures.json; upload both in a stable per-leg artifact name. Run name set to “C# Matrix Tests” for scheduled and manual full‑matrix runs.
• report-failures.yml: New workflow_run reporter that triggers after the "C# tests" workflow, but only for schedule/workflow_dispatch runs that failed. Aggregates artifacts across all legs and posts to a single rolling issue.
• Aggregation details: Prefer failures.json; fall back to TRX parsing via xmlstarlet. Build a single Markdown section per failing leg with a concise header (framework, server, host) and a bulleted list of failed tests with short error excerpts. Totals across all legs are summarized at the top. If the pipeline failed with zero detected test failures, report as an infrastructure/job failure.
• Permissions: Least privilege (actions: read, contents: read, issues: write).
• Reliability: Artifacts uploaded with if: always() so reporter can fetch even on failures; reporter guards comment duplication using the workflow_run id marker.
• Build hygiene: Suppress NETSDK1138 in benchmarks to keep CI green on net6.0.

Notes

• Current reporter scope is schedule/workflow_dispatch full‑matrix runs; it ignores PR/push runs by design to reduce noise.
• Follow‑ups (optional, separate PR): auto-close the rolling issue on recovery, chunking for very large outputs, and widening branch scope after bedding-in.

Validation

• YAML lint‑clean locally; reporter exercised in a feature branch with synthetic artifacts. Next nightly will validate end‑to‑end; manual full‑matrix run can also be triggered.