Make the ComponentTests checker tests asynchronous

[xperiandri]

Description

Test-only change. The checker tests in FSharp.Compiler.ComponentTests that blocked on Async.RunSynchronously now return Task and bind the FCS Async results inside task { }, so they no longer park an xunit worker thread while the checker runs, and they read like the Task-based tests already in the project.

• FSharpChecker/SemanticClassificationRegressions.fs — getClassifications, classificationsForIdent and expectUnionCaseClassifications are asynchronous; every test binds them with let!. Source snippets keep their exact text and columns, only the code around them moved.
• FSharpChecker/FindReferences.fs — findRefsInSource, testFindRefsInSource and singleFileCheckerWithName are asynchronous; the tests that call them return the resulting Task.
• FSharpChecker/CommonWorkflows.fs, FSharpChecker/TransparentCompiler.fs (Fuzzing, getParseResult), CompilerDirectives/Line.fs, Miscellaneous/TestUtilities.fs, TypeChecks/Graph/CompilationFromCmdlineArgsTests.fs — the same, one site each.
• CompilerService/AsyncMemoize.fs — waitUntil, expect and assertTaskCanceled return Async/Task and the tests await them; .Result/.Wait() on the tasks those tests create became let!/do! too. Stress test is untouched: its Task.Wait with a timeout is the point of the test.
• FSharp.Test.Utilities/ProjectGeneration.fs — singleFileChecker returns Async; its only callers are the files above.

Left as is: Async.RunSynchronously inside the F# source strings that the tests compile or run (the behaviour under test), TypeChecks/Graph/Utils.fs's parseSourceCode (called from record literals in 30 places across the trie/graph tests; a separate change), and FSharp.Compiler.Service.Tests, which has a few hundred more sites.

Checklist

• Test cases added — no new tests; the touched modules were run locally after the change.
• Performance benchmarks added in case of performance changes — n/a.
• Release notes entry updated — tests only, no release note.

[github-actions]

✅ No release notes required

[xperiandri]

Reviewed cd3849f as a full diff plus the untouched callers of every helper whose shape changed.

Verdict: the conversion is sound; nothing blocking. Two inline remarks — one fold-the-duplicate (non-blocking, but this PR is the natural place for it), one nit.

What was checked:

• Every caller of a helper that changed shape (singleFileChecker, parse, getParseResult, findRefsInSource, getClassifications, classificationsForIdent, waitUntil/expect/assertTaskCanceled) is converted, and none exists outside FSharp.Compiler.ComponentTests. Stress test and the two pre-existing task tests in AsyncMemoize.fs use none of them, so leaving them alone is right.
• let!/do! on an Async inside task { } goes through Async.StartImmediateAsTask with the default cancellation token — the same token and exception propagation Async.RunSynchronously had — so there is no semantic drift, including the try … finally around Environment.CurrentDirectory in CompilationFromCmdlineArgsTests (both theories are skipped anyway).
• Source snippets in triple-quoted strings keep their columns; the UTF-8 BOM on the five files that had one is preserved; git diff --check is clean.
• Ran the touched modules locally against a Debug/net11.0 build of this commit: AsyncMemoize, SemanticClassificationRegressions, FindReferences, CommonWorkflows, CompilerDirectives.Line, Miscellaneous.TestUtilities — 128 passed, 2 skipped, 0 failed. Fuzzing is Skipped on main, so its conversion is compile-verified only.

Non-blocking observation: the explicit : Task return annotation is new to this project — the existing task { } tests, including two in AsyncMemoize.fs and those in TransparentCompiler.fs, rely on the inferred Task<unit>. It is a good guard (an accidentally returned Async<unit> becomes a compile error instead of a value xunit has no way to await), so I would keep it; the two neighbours in AsyncMemoize.fs could get it too for uniformity, or not — either is fine.