Fix enum wire values

[tolgahan-arikan]

Summary

• use explicit RIDL enum values for C string enum conversion helpers instead of enum identifiers
• keep non-string enums using their existing identifier-based behavior
• add regression coverage for explicit string enum round-tripping

Why

The C generator was emitting enum names like Ethereum and Native from *_to_string / *_from_string even when the schema defined explicit wire values like ethereum and native. That breaks JSON encoding and decoding for generated clients. The fix is scoped to string enums so numeric enum behavior stays unchanged.

Validation

• go test ./...