Remove obsolete MSTest 5 and Microsoft.Testing.Platform 3 compatibility APIs and configuration aliases

Scope and rationale

Track the six approved major-version removal groups for MSTest 5.0 / Microsoft.Testing.Platform (MTP) 3.0. The reviewed source and implementation baseline is 471a807cf8182e0bcd0d9967c8dbf002c04945d2. This is planned breaking cleanup, not authorization to remove other compatibility surfaces.

Intended breaks and migrations

• MSTest TestResult unused properties: remove InnerResultsCount, DatarowIndex, and ReturnValue from Microsoft.VisualStudio.TestTools.UnitTesting.TestResult. They are already obsolete-as-error and have no supported effect. Custom result producers and reflection/binary consumers must stop accessing them; no replacement is needed for these unused values. Other result properties and .NET Framework remoting remain supported.
• MTP internal Helpers.AppVersion: remove the obsolete compatibility type and DefaultSemVer. Migrate the remaining in-repository test to PlatformVersion.Version. Friend/source consumers that use this internal shim must also migrate. Do not remove unrelated extension version helpers or friend grants.
• MTP TestApplication.CreateServerModeBuilderAsync: remove the obsolete public method. Call TestApplication.CreateBuilderAsync with exactly one explicit --server argument. The removed helper injected/deduplicated the server switch, so changing only the method name is not an equivalent migration. Already-compiled callers need an upgraded/recompiled consumer.
• MTP CancelledTestNodeStateProperty: remove the obsolete public in-process test-node state, its public/internal API records through explicit removal entries, and branches/suppressions/tests that exist only for that state. Test frameworks should throw OperationCanceledException carrying MTP's supplied cancellation token instead. Audit terminal/TRX/HTML/CI reporters, category/result aggregation, JSON-RPC, dotnet-test IPC, Retry, HangDump and CrashDump. Update the linked server-client source package and its tests as well as the platform. Preserve functional cancellation, fault observation, and older-peer wire states unless a separately established protocol contract requires a targeted migration. Extensions that still bind to the removed CLR type must upgrade; identify any exact compatibility dependency rather than dropping all old-extension support.
• MTP diagnostic environment aliases: remove TESTINGPLATFORM_DIAGNOSTIC_OUTPUT_FILEPREFIX and TESTINGPLATFORM_DIAGNOSTIC_FILELOGGER_SYNCHRONOUSWRITE, their fallback reads, deprecation warnings, and directly dependent resources/tests. Set TESTINGPLATFORM_DIAGNOSTIC_FILE_PREFIX and TESTINGPLATFORM_DIAGNOSTIC_SYNCHRONOUS_WRITE respectively. The old names will no longer affect configuration; supported CLI/environment precedence must remain intact.
• MSTest flat ordering configuration alias: remove the fallback and warning for mstest:orderTestsByNameInClass. Use mstest:execution:orderTestsByNameInClass in testconfig.json. The removed flat key will no longer enable ordering; defaults and the nested setting remain unchanged.

Consumer and compatibility boundaries

The API removals intentionally break source/binary consumers of those exact APIs. Keep shipped API history intact and record approved removals with *REMOVED* entries in the appropriate unshipped public/internal baselines. Do not silently widen this to other shims, defaults, or protocol changes.

ForwardCompatibilityTests currently exercises older 2.2.1 extensions and MSBuild 2.3.0; TelemetryForwardCompatibilityTests exercises telemetry 2.0.0 on .NET Framework. Preserve this compatibility coverage wherever it does not depend on the specifically removed state. Establish/document the exact dependency of an old consumer that must upgrade instead of deleting unrelated compatibility tests.

Explicitly excluded: the old orchestrator facade; parameterless IProcess.WaitForExitAsync; compatibility-only polyfill exports; old-engine friend grants; --no-progress / --no-ansi; HangDump %p; legacy OpenTelemetry output; serialization constructors; HasExitedGracefully renaming; IsTestingPlatformApplication default changes; UWP/WinUI package splitting; assertion misuse guards; VSTest interface implementations; .NET Framework remoting; Jsonite; necessary polyfills; functional cancellation/fault observation.

Delivery and validation

Update directly related migration/changelog documentation, preserve UTF-8 BOM in C# sources, and regenerate removed localization entries via each owning project's UpdateXlf target. Use the repository-pinned SDK/build scripts. Keep focused consumer coverage for supported configuration, explicit server mode, token cancellation, linked source-only clients and relevant old extensions. Pack before package-consuming acceptance runs; identify the actual packages/binaries consumed. Distinguish executed behavior evidence from compilation, static predictions, skipped/zero-test runs and unavailable platform checks. No pull request is requested by this issue creation.

Verified extension migration dependencies

Removing the approved internal AppVersion shim is also a binary break for older friend assemblies, not just an in-repository test migration. Package metadata and a real packed consumer established these specific dependencies:

These are tested package combinations, not a universal or monotonic minimum-version promise for all 2.x extensions. The original fixture executed and failed with TypeLoadException naming the two removed types. The narrowly migrated mixed-version fixture passed, asserting the TRX result's exact name, Passed outcome, and total/executed/passed counts of one. The unchanged telemetry 2.0.0 .NET Framework canary also passed. Local package-consuming validation used MSTest 5.0.0-dev and MTP 3.0.0-dev; it is not CI or published-release certification. Older-peer JSON-RPC canceled responses, cancellation requests, reserved IPC state values, and historical report-journal states remain supported.