Revert Roslyn diagnostics workaround (fix #16360)

[T-Gro]

Fixes #16360

Summary

Reverts the workaround introduced in #15982 that was causing F# diagnostics to appear doubled in VS tooltips. The original workaround was added because Roslyn wasn't surfacing diagnostics properly (#15972), but that issue has since been resolved on the Roslyn side.

Changes

• DocumentDiagnosticAnalyzer.fs: Restore the original logic that reports parse diagnostics only in the syntax pass and excludes them from the semantic pass (via �rrors.ExceptWith(parseResults.Diagnostics)). This prevents the same diagnostic from showing up twice.
• DocumentDiagnosticAnalyzerTests.fs: Add helper getSyntaxAndSemantic and six new tests verifying that diagnostics are not duplicated across syntax/semantic passes. Update existing test assertions from expecting 2 duplicated errors to expecting 1.
• Release notes: Added entry for the fix.

[T-Gro]

LGTM – clean revert of the workaround now that Roslyn handles the diagnostics correctly. The ExceptWith logic is the right way to deduplicate, and the new tests cover the key scenarios well.

Two minor suggestions (non-blocking):

1. Rename the HACK methods: Now that the workaround is gone, the TODO comments explaining the HACK names were removed, but the method names still carry the _HACK_PLEASE_REFER_TO_COMMENT_INSIDE suffix. Consider renaming them back to their original names (e.g. VerifyDiagnosticBetweenMarkers, VerifyErrorBetweenMarkers, VerifyErrorAtMarker) since the hack is no longer needed.

2. Warning not duplicated across passes test: The source let x = 42 at module level doesn't produce any warnings (it's a public binding), so allWarnings is always empty and the assertion trivially passes. Consider using source that actually emits a warning (e.g. open System without using anything from it, or a private unused binding) to make the test meaningful.

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path	Release notes path	Description
vsintegration/src	docs/release-notes/.VisualStudio/18.vNext.md	No current pull request URL (#19812) found, please consider adding it