[JAVA] [PERF] Use maps for lookups in enum fromValue methods

Is your feature request related to a problem? Please describe.

The current modelEnum.mustache templates implementation for fromValue for the JVM has an O(n) worst case.

Describe the solution you'd like

I want the fromValue to use a map to retrieve the enum value in constant time.

Describe alternatives you've considered

Possible alternatives are

1. Ignore this request. The implementation and possible impacts are too troublesome for the project.
2. Use hashmaps/treemaps for the lookup. I have opened a PR with benchmarks and the implementation targeting Java -> https://github.com/OpenAPITools/openapi-generator/pull/22995

Additional context

I have run jmh benchmnarks, which yield the following results (MacOS ARM 26;JMH version: 1.37;JDK 11.0.22, OpenJDK 64-Bit Server VM, 11.0.22+7-LTS)

Benchmark                                                        (inputSize)   Mode  Cnt         Score         Error  Units
EnumLookupJmhBenchmark.hashMapLookupCaseInsensitive                    50000  thrpt    5  25144177.892 ±  820137.701  ops/s
EnumLookupJmhBenchmark.hashMapLookupWithOptional                       50000  thrpt    5  99265713.896 ± 2460266.752  ops/s
EnumLookupJmhBenchmark.hashMapLookupWithoutOptional                    50000  thrpt    5  99777930.270 ± 2376641.407  ops/s
EnumLookupJmhBenchmark.linearLookup                                    50000  thrpt    5  24858482.938 ±  367926.448  ops/s
EnumLookupJmhBenchmark.linearLookupCaseInsensitive                     50000  thrpt    5  12758767.698 ±  667129.809  ops/s
EnumLookupJmhBenchmark.treeMapLookupCaseInsensitiveWithOptional        50000  thrpt    5  14069454.341 ±  331263.365  ops/s

The benchmarks can be run by anyone, the command is provided.

Start by reviewing the linked pull request 22995, then inspect the JVM implementation in the modelEnum.mustache template and the EnumLookupJmhBenchmark results described here. Done means the generated fromValue lookup uses the agreed map-based approach and the relevant benchmarks or tests validate the change without regressions.