Fix #16360: stop duplicating editor diagnostics (revert Roslyn workaround)

[T-Gro]

Fixes #16360.

Diagnostic messages produced by the F# editor analyzer were appearing twice in Visual Studio (once per analyzer pass), and the long-standing workaround for that (#15982) was suppressing duplicates in the analyzer itself.

With the underlying Roslyn duplication no longer occurring, the workaround is unnecessary. Removing it:

• restores a single diagnostic per source location (tooltips, error list, squigglies),
• lets the test helpers go back to asserting 1 diagnostic instead of 2,
• removes the _HACK_PLEASE_REFER_TO_COMMENT_INSIDE test helpers, which are now identical to their normal counterparts.

A failing test was added first to lock in the expected single-diagnostic behavior, then the workaround was reverted, then the duplicated helpers were collapsed.

[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]

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