Avoid double TestCase construction when filtering during discovery (#9568)

[Evangelink]

Fixes #9568.

Problem

When a test-case filter is active, every matched UnitTestElement was converted to a VSTest TestCase twice during discovery:

1. inside TestElementFilter.Matches — to evaluate the filter (VSTest's MatchTestCase requires a TestCase), and
2. inside the discovery sink (HostDiscoverySink / TestCaseDiscoverySink) — to report the matched test.

For large filtered suites this doubled TestCase allocations (including the Id-hashing work) for every matched test. Purely redundant — no behavior/correctness impact.

Fix

Added UnitTestElement.GetOrCreateHostTestCase() — a lazily-cached host TestCase distinct from the always-fresh ToTestCase(). The discovery filter and both discovery sinks now reuse the cached instance, so a matched element is converted to a TestCase exactly once.

• ToTestCase() stays fresh on every call, so mutation-sensitive callers (e.g. UnitTestElementTests, which mutate → rebuild → assert) are unaffected.
• The neutral ITestElementFilter / IUnitTestElementSink interfaces keep their UnitTestElement-based signatures — the in-flight platform-agnostic refactor is not perturbed.
• The cache is reset in Clone / CloneWithUpdatedSource (the latter changes the source, hence the Id) and excluded from serialization via [NonSerialized] under NETFRAMEWORK.

Only 4 files changed (+28 / −3).

Verification

• Full Debug build: 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net9.0): 893/893 passed (filter, sink, discoverer, and UnitTestElement Id tests).
• MSTest.IntegrationTests TestCaseFilteringTests: 4/4 passed — the end-to-end regression guard locking FQN + Id filtering across discovery and execution.
• MSTestAdapter.UnitTests MSTestDiscovererTests: 16/16 passed.

[Copilot]

Pull request overview

This PR eliminates a redundant TestCase allocation that occurred for every matched test during discovery. Previously, when a test-case filter was active, each UnitTestElement was converted to a VSTest TestCase twice: once inside TestElementFilter.Matches (to evaluate the filter via VSTest's MatchTestCase) and again in the discovery sink (to report the test). It fits into the ongoing platform-agnostic PlatformServices refactor (follow-up to #9567/#9568) without perturbing the neutral ITestElementFilter/IUnitTestElementSink interfaces.

The approach adds a lazily-memoized host TestCase (GetOrCreateHostTestCase()) alongside the always-fresh ToTestCase(). The filter and both discovery sinks now share the cached instance, so a matched element is converted exactly once. The cache is reset on Clone/CloneWithUpdatedSource and marked [NonSerialized] under NETFRAMEWORK, which is important because UnitTestElement[] is marshalled across the AppDomain boundary during execution.

Changes:

• Added UnitTestElement.GetOrCreateHostTestCase() memoizing a host TestCase, with cache invalidation in both clone paths and [NonSerialized] under NETFRAMEWORK.
• Updated TestElementFilter.Matches and both discovery sinks (HostDiscoverySink, TestCaseDiscoverySink) to reuse the cached instance.
• ToTestCase() behavior is unchanged (still fresh per call), so mutation-sensitive callers are unaffected.

Show a summary per file

File	Description
src/Adapter/MSTestAdapter.PlatformServices/ObjectModel/UnitTestElement.cs	Adds the _hostTestCase cache field, GetOrCreateHostTestCase(), [NonSerialized] guard, and cache reset in both clone methods.
src/Adapter/MSTestAdapter.PlatformServices/TestMethodFilter.cs	TestElementFilter.Matches now reuses the cached host TestCase instead of building a throwaway one.
src/Adapter/MSTestAdapter.PlatformServices/Services/UnitTestElementSinkExtensions.cs	HostDiscoverySink.SendTestElement forwards the cached TestCase to the VSTest sink.
src/Adapter/MSTestAdapter.PlatformServices/Execution/TestCaseDiscoverySink.cs	Internal execution discovery sink collects the cached TestCase.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Medium