[TESTS-FOR-ALL-GENS] - refactor: replace Collectors.toMap with custom collector for case-insensitive file name ordering to simplify lookup in the table while debugging tests.

[Picazsoo]

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Summary by cubic

Refactored tests to use a shared collector that maps generated files by name into a deterministically ordered TreeMap. This makes lookups predictable and debugging easier, with no changes to generator behavior.

• Refactors
  • Added TestUtils.collectToCaseInsensitiveOrderedCaseSensitiveKeyMap to build a TreeMap ordered case-insensitively (then case-sensitively).
  • Replaced Collectors.toMap(File::getName, ...) with the new collector across tests.
  • Duplicate file names now fail fast with IllegalStateException.
  • Removed now-unneeded Function/Collectors imports in updated tests.

^{Written for commit c531df4. Summary will update on new commits.}