feat(test-sdk): expose SDK-owned coverage settings path

[xshaheen]

Summary

External Microsoft Testing Platform module runners can now resolve the Test SDK's packaged coverage denominator policy instead of maintaining a consumer-owned copy. Project-evaluated runs use the same public HeadlessCoverageSettingsPath property when the SDK injects --coverage-settings, so both execution modes share one policy owner.

The policy keeps every intentional generated-code, migration, and test-assembly exclusion and adds a cross-platform obj/**/generated/** defense for compiler and source-generator output with unconventional filenames. Semantic package tests prove the new rule does not exclude ordinary handwritten source or partial classes.

Consumption contract

• Resolves an absolute, existing packaged settings path in project-SDK, global.json SDK, additional-SDK, and PackageReference consumption.
• Injects --coverage and exactly one property-backed --coverage-settings argument when EnableCodeCoverage=true.
• Documents why --test-modules cannot receive SDK-injected project arguments and shows external tooling how to query, validate, quote, and pass the SDK-owned path.
• Keeps explicit MSBuild values authoritative while providing the packaged path as the default.

Validation

• CSharpier repository check: passed (24 files checked).
• Restore: passed; all projects up to date.
• Release build: passed with 0 warnings and 0 errors.
• Release pack: passed; six 0.2.2-preview.0.5 packages produced.
• Focused packed-package contracts: 5 passed, 0 skipped, 0 failed.
• Full packed-package suite: 159 total, 157 passed, 2 skipped, 0 failed in 9m 02s.
  • Skips are the Windows-only WPF and Windows Forms consumer builds on macOS.
• Packed Headless.NET.Sdk.Test archive inspected: build/SupportTestProjects.targets and configurations/default.runsettings are present, with the public property, property-backed argument, and approved exclusions.

Follow-up boundary

The downstream framework module runner still needs to resolve this property, retain its Cobertura regression verifier, and remove its duplicated eng/coverage.runsettings. This PR does not modify Headless Framework, bump an SDK version, publish packages, release, or merge.

Related: xshaheen/headless-framework#814

Post-Deploy Monitoring & Validation

There is no production runtime deployment. For the first downstream adoption, query the property after restore and require a fully qualified existing file before starting the module runner. A missing/empty path or a Cobertura denominator change is the regression signal; the SDK and framework PR authors own that adoption check.