Prevent duplicate rule ID suppression errors in `-Fix` and DSC scenarios (#2181)

* Enhance suppression error handling in ScriptAnalyzer and add tests for unapplied suppression errors

* Skip new -Fix suppression test under Library Usage harness

The `Library Usage` describe block in `LibraryUsage.tests.ps1` (only
active on Windows PowerShell 5.1, since it's gated `-Skip:$IsCoreCLR`)
re-runs `RuleSuppression.tests.ps1` against a hand-rolled
`Invoke-ScriptAnalyzer` wrapper that drives the analyzer as a .NET
library. That wrapper plugs in `PesterTestOutputWriter`, whose
`WriteError` is intentionally a no-op:

    public void WriteError(ErrorRecord error)
    {
        // We don't write errors to avoid misleading
        // error messages in test output
    }

So the unapplied-suppression `ErrorRecord` we now emit during the final
`-Fix` pass never reaches `-ErrorVariable`, and `$fixErr | Should
-HaveCount 1` fails with "Expected a collection with size 1, but got an
empty collection". The behaviour itself is correct - the assertion is
just unobservable through this test harness.

Mark the new `It` block `-Skip:$testingLibraryUsage`, matching the
existing pattern already used by the `Bad Rule Suppression` and
`External Rule Suppression` contexts in the same file for the same
reason. The regular pwsh and WinPS runs of `RuleSuppression.tests.ps1`
(which `[+]` in the failing CI log) continue to exercise the assertion.

The new `UseDSCResourceFunctions.tests.ps1` test isn't dot-sourced by
`LibraryUsage.tests.ps1`, so it doesn't need the same guard.

Drafted by Copilot (Claude Opus 4.7).

* Add copilot review suggestions

---------

Co-authored-by: Andy Jordan <2226434+andyleejordan@users.noreply.github.com>