Switch test reporting to dorny/test-reporter

[Copilot]

Replace EnricoMi/publish-unit-test-result-action@v2 with dorny/test-reporter@v2 for test result reporting in the Tests workflow.

• Inline reporting: dorny/test-reporter runs directly in the test job using the TRX files already on disk, eliminating the separate publish-test-results job that had to download artifacts
• Per-OS check runs: Each matrix entry gets its own named check run (Test Results (ubuntu-latest), Test Results (macos-15-intel)) with per-test annotations and inline failure markers
• Permissions: Dropped pull-requests: write (was only needed by EnricoMi for PR comments); checks: write is sufficient for dorny

- name: Test Report
  uses: dorny/test-reporter@v2
  if: (always() && !cancelled())
  with:
    name: Test Results (${{ matrix.os }})
    path: ./*.Tests/TestResults/*.trx
    reporter: dotnet-trx

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