#25299 refactor: replace direct enum checks with enum utility methods

Replaces reoccurring schema enum checks with util functionality from the ModelUtils.

Many locations do schema.getEnum() != null when they would most likely not accept an empty enum array. I have left these as is since it is out of scope for this change to investigate whether they would permit that scenario or not.

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Summary by cubic

Replaces repeated inline getEnum() checks with new utility methods in ModelUtils, consolidating enum detection logic across codegen.

• Adds isEnum (loose check: enum list present, may be empty) and hasSingleEnum; isEnumSchema remains the strict check (non-null, non-empty). The now-unused hasEnum helper is removed.
• Updates around 20 call sites across generators, examples, and OAS 3.1 utilities to use the helpers, preserving each site's original semantics.
• Leaves sites checking only getEnum() != null where an empty enum might not be acceptable unchanged, as verifying that is out of scope.