Fix #668: JSON codec built from auto-derived schema fails for enumerat

[JoshKappler]

Summary

The JSON codec's enumDecoder used isInstanceOf[CaseClass0[_]] checks with unsafe asInstanceOf casts to determine simple enum handling, which was misaligned with the encoder's simpleEnum annotation check. This caused a ClassCastException when an enumeration contained intermediate sealed traits (producing Schema.Enum schemas instead of Schema.CaseClass0). The fix aligns the decoder with the encoder by using the simpleEnum annotation check and replaces unsafe casts with the existing type-safe constructEnumCase helper.

Changes

• zio-schema-json/shared/src/main/scala/zio/schema/codec/JsonCodec.scala: Changed enumDecoder to check the simpleEnum annotation (matching enumEncoder behavior) instead of checking whether all cases are CaseClass0 instances. Replaced unsafe asInstanceOf[Schema.CaseClass0[...]] casts with calls to the existing constructEnumCase helper, which safely handles both CaseClass0 and Schema.Enum cases.

Testing

• Added JsonCodecSpec668 with 10 test cases covering enumerations with intermediate types — all pass on Scala 2.13 and Scala 3.3.7.
• Full zioSchemaJsonJVM/test suite passes (298 tests on Scala 2.13, 304 on Scala 3).
• Full zioSchemaDerivationJVM/test suite passes (91 on Scala 2.13, 101 on Scala 3).

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/claim #668

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[JoshKappler]

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