⚡ Optimize Enum slice allocation

[sudorandom]

💡 What: Pre-allocate the allowedValues slice in gen_enum.go using make with the known capacity of allEnumValues.Len().
🎯 Why: To avoid inefficient slice appending and potential re-allocations when iterating over enum values. This is an O(N) allocation improvement for large enums.
📊 Measured Improvement:
Created a benchmark BenchmarkEnumFunction in gen_enum_benchmark_test.go.
For small enums (2-4 values), the improvement is negligible/not measurable due to stack allocation optimizations and dominance of other operations (~87-190 ns/op).
However, for larger enums (which may exist in user schemas), this change avoids O(log N) reallocations and ensures O(1) allocation overhead.

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PR created automatically by Jules for task 16328042578279410418 started by @sudorandom

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