Move test-case filter parsing to adapter boundary (Phase 6d-1)

[Evangelink]

Phase 6d-1 of the PlatformServices platform-agnostic initiative

Part of removing the VSTest object model (Microsoft.TestPlatform.ObjectModel) dependency from MSTestAdapter.PlatformServices.

This phase moves the test-case filter parsing to the adapter boundary. TestMethodFilter (which parses the VSTest ITestCaseFilterExpression from IRunContext/IDiscoveryContext.GetTestCaseFilter) moves out of PlatformServices into MSTest.TestAdapter, and the engine/discoverer receive an injected neutral ITestElementFilter via a new ITestElementFilterProvider abstraction.

What changed

• New ITestElementFilterProvider (PlatformServices.Interface): ITestElementFilter? GetTestElementFilter(IAdapterMessageLogger, out bool filterHasError).
• Moved TestMethodFilter (+ nested TestElementFilter) → MSTest.TestAdapter (class body byte-identical); added a boundary TestElementFilterProvider that closes over the VSTest context.
• The boundary (MSTestExecutor, MSTestDiscoverer — which the MTP MSTestBridgedTestFramework also routes through) builds new TestElementFilterProvider(context) and injects it into TestExecutionManager.RunTestsAsync/ExecuteTestsAsync and UnitTestDiscoverer.DiscoverTests.
• The engine/discoverer invoke the provider at the exact same call sites they previously built the filter (per source), so parse-error reporting keeps the same timing and per-source count.

No behavior change

TestElementFilter.Matches still does element.ToTestCase() (byte-for-byte; the #9568 no-round-trip optimization is deferred as its own follow-up). This removes ITestCaseFilterExpression / GetTestCaseFilter / MatchTestCase / the VSTest filter TestProperty set from PlatformServices code. IRunContext/IDiscoveryContext remain only for deployment + settings extraction (removed in 6d-2).

Verification

• Full build green across all TFMs (net462, net8.0, net9.0, UWP, WinUI).
• MSTestAdapter.PlatformServices.UnitTests: 897 (net8.0) / 935 (net462); MSTestAdapter.UnitTests: 21; MSTest.IntegrationTests: 47 pass / 1 skip — including TestCaseFilteringTests, the out-of-proc filter regression net.
• Expert MSTest/MTP reviewer: no material findings; definitive byte-for-byte verdict on the per-source parse-error timing/count and the null-provider equivalence.

Base / stacking

• Stacked on Phase 6c2 (Neutralize run-settings input in PlatformServices host layer (Phase 6c2) #9590). Base = dev/amauryleve/vstest-decoupling-settings; review/merge after Neutralize run-settings input in PlatformServices host layer (Phase 6c2) #9590. When 6c2 merges into vstest-decoupling-base, this PR is rebased onto the updated base (diff is 6d-1 only, 10 files).

Remaining work

• 6d-2: remove IRunContext/IDiscoveryContext from PlatformServices entirely (deploy-context + settings extraction to the boundary).
• Avoid double TestCase construction when filtering during discovery/execution (platform-agnostic refactor follow-up) #9568: filter no-round-trip optimization (only if provably byte-for-byte).
• 6e: relocate remaining bridges + the deep UnitTestElement↔TestCase conversion + EngineConstants.
• 7: remove the Microsoft.TestPlatform.ObjectModel PackageReference + guard test.