Prevent duplicate rule ID suppression errors in -Fix and DSC scenarios

[liamjpeters]

PR Summary

• When using -Fix, only emit errors for unused rule suppressions IDs on the final pass - not intermediate Fix() passes.
• Make the two DSC branches mutually exclusive (skip the module-layout branch when the class-based branch already handled the AST). Prevents multiple copies of unused rule suppression ID error.

Added regression tests covering these 2 scenarios.

Fixes #2177

PR Checklist

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