Ruolin Fan

[Coral-Schema] Disambiguate nested record namespaces with numeric suffixes

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### What changes are proposed in this pull request, and why are they necessary? - Fix Avro namespace collisions when a parent schema contains multiple fields that reference nested records...

[Coral-Schema] handle single-element unions when the supplied column schema does not specify <uniontype>

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### What changes are proposed in this pull request, and why are they necessary? When Avro schemas define single-element unions (e.g., `["string"]` for primitives, `[{"type":"record",...}]` for array items/map values), the...

### What changes are proposed in this pull request, and why are they necessary? This PR fixes a bug in `ToLowercaseSchemaVisitor` where field names within complex default values were not...

### What changes are proposed in this pull request, and why are they necessary? ### How was this patch tested?