#25066 fix(java): validate string enum values excluded by not

Fixes #25065

What does this PR do?

Fixes Java okhttp-gson validation for string properties that use not with an enum.

For a schema such as:

kind:
  type: string
  not:
    enum:
      - known

the generated validateJsonElement method previously checked only that kind was a string and did not enforce the value excluded by not.

This caused oneOf schemas that rely on not: enum to become ambiguous. For example, {"kind":"known"} could incorrectly match both the schema that explicitly allows "known" and the schema that excludes it.

Implementation

• Reads string enum values from the property's composedSchemas.not metadata during Java client model-property post-processing.
• Exposes those excluded values to the okhttp-gson POJO template.
• Generates validation that rejects matching excluded string values.
• Keeps the change intentionally scoped to Java okhttp-gson and string enums under not.

Tests

Added an OpenAPI 3.1 regression fixture with:

• Known.kind: enum: [known]
• Other.kind: not: { enum: [known] }
• Union: oneOf: [Known, Other]

Added a Java client generation test that verifies the generated Other validation:

• checks the actual kind property;
• rejects "known";
• does not incorrectly validate a synthetic not_schema property.

Targeted test:

JavaClientCodegenTest#testOneOfNotEnumValidation

passes successfully.

---

Summary by cubic

Fixes Java okhttp-gson validation so string properties using not with an enum reject excluded values, resolving ambiguity in oneOf schemas that rely on not: enum (issue #25065).

Previously validateJsonElement only checked that the property was a string, so {"kind":"known"} could match both a schema allowing "known" and one excluding it. The generator now reads the excluded values from composedSchemas.not, exposes them via a vendor extension, and generates validation that throws an IllegalArgumentException for disallowed values.

The change is scoped to Java okhttp-gson string enums under not, and adds a regression fixture plus a generation test covering the oneOf scenario, including enum references.