Prepare affected-test selection rollout

[Evangelink]

Summary

Prepares testfx to adopt the experimental Microsoft.Testing.Platform affected-test workflow from dotnet/sdk#55574, using the composable filter-provider support already merged in testfx#10235.

The rollout is intentionally disabled. The SDK change has not flowed into this repository's pinned SDK, and the affected-test extension package and public storage schema are not available yet. Existing test runs therefore keep their current command and behavior.

Prepared now

• adds repository-specific test.affectedTests change and instrumentation scopes to global.json;
• prepares trusted main Windows Debug for future --collect-test-map collection;
• prepares PR Windows Debug for future --affected-tests selection;
• keeps both call sites at enableAffectedTests: false;
• scopes DOTNET_CLI_ENABLE_AFFECTED_TESTS=1 to the inactive affected-test branches;
• prevents pipelines from setting the SDK-owned DOTNET_CLI_TEST_AFFECTED_TESTS_MODE marker;
• adds a validation script and CI check for disabled/activated state consistency, argument construction, secret-free configuration, and gate isolation;
• documents storage, activation, validation, and rollback.

Intentionally gated

The storage property is intentionally absent from global.json rather than inventing a provider name, package, credential, or private endpoint. The intended CI design is Azure Blob Storage as the durable cross-run map, authenticated through secure pipeline facilities, with Azure DevOps artifacts used only for non-secret diagnostics or snapshots.

Activation requires:

1. an SDK build containing Add affected test selection to dotnet test dotnet/sdk#55574;
2. a publicly available affected-test extension package flowed through normal dependency management;
3. the package's published Azure Blob storage schema and secure pipeline authentication;
4. a successful compatible mapping collection before PR selection becomes required.

Rollout and rollback

After the prerequisites flow, add the documented storage configuration, enable collect on trusted main, validate the produced map and diagnostics, then enable run for PR Windows Debug. Validate documentation-only, narrowly affected, force-all, missing/incompatible-map, fork-PR, and collection-failure scenarios before making selection required.

Rollback is explicit and immediate: set enableAffectedTests back to false. The unchanged full test command remains the disabled branch.

Validation

• Debug repository build: 0 warnings, 0 errors.
• Disabled, collect activation, missing-storage, job-level gate leakage, and SDK child-marker leakage scenarios passed.
• global.json and modified YAML parse successfully.
• PSScriptAnalyzer and markdownlint pass.
• git diff --check passes.
• Four fresh bug-focused review rounds completed; fixes covered activation deadlock and environment-variable leakage bypasses, and the final review found no significant issues.

[Copilot]

Pull request overview

Prepares an intentionally disabled rollout of affected-test selection for Microsoft.Testing.Platform CI.

Changes:

• Adds affected-test scopes and gated pipeline modes.
• Adds rollout validation and safety checks.
• Documents activation, storage, validation, and rollback.

Show a summary per file

File	Description
global.json	Defines affected-test scopes.
azure-pipelines.yml	Configures disabled collect/run call sites and validation.
eng/pipelines/steps/test-windows-debug-coverage.yml	Adds gated affected-test commands.
eng/validate-affected-tests.ps1	Validates rollout configuration.
docs/affected-test-selection.md	Documents rollout procedures.
docs/README.md	Links the new guide.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 3
• Review effort level: Balanced