#25008 fix(java): do not map 'format: enum' schemas to 'Enum'

…en (fixes #24950)

fixes #24950

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In PR #24812, typeMapping.put("enum", "Enum") was added to AbstractJavaCodegen to resolve common discriminator types for oneOf interface methods across enum subtypes. However, because DefaultCodegen.getSchemaType checks typeMapping for format matches, schemas specifying format: enum were mapped to type "Enum", causing invalid Java code generation (private Enum value; and invalid Enum.valueOf(...) calls).

This PR:

• Introduces protected String getCommonEnumType() in DefaultCodegen (overridden in AbstractJavaCodegen to return "Enum").
• Replaces direct typeMapping.get("enum") lookups in discriminator resolution with getCommonEnumType().
• Removes typeMapping.put("enum", "Enum") from AbstractJavaCodegen, preventing the format collision.
• Adds a unit test in JavaClientCodegenTest verifying that schemas with format: enum generate valid String backing fields and constructors without invalid Enum.valueOf(...) or imports.
  cc: @wing328

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Summary by cubic

Fixes Java code generation for schemas with format: enum so they no longer produce invalid Enum types.

• Adds a getCommonEnumType() hook in DefaultCodegen; the base implementation keeps the typeMapping["enum"] lookup and falls back to the mapped object type when absent.
• Overrides the hook in AbstractJavaCodegen to return "Enum", and removes typeMapping.put("enum", "Enum") so format: enum strings are no longer resolved to Java's Enum.
• Adds a unit test verifying format: enum schemas generate String-backed enums without Enum.valueOf(...), invalid Enums fields, or Enum imports.