[Java] Fix missing underscores for PascalCase enum values #4837

[renatomameli]

Resolves: #4837

Addresses an issue with the conversion of enum values to valid variable names. When the enum value is a string and contains multiple words without separators (e.g., TestCase), underscores between the words were previously missing (e.g., "TESTCASE").

To address this issue, the existing underscore method is now applied to the value before all characters are converted to uppercase letters. The underscore method converts CamelCase to SCREAMING_SNAKE_CASE while preserving the case of individual words (e.g., TestCase becomes TEST_CASE)

Corresponding tests have been added.

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[renatomameli]

@lwlee2608 @martin-mfg

[renatomameli]

@wing328 samples updated