AVRO-3671: [C#] Preserve specific enum types

[sadicangel]

What is the purpose of the change

Fix AVRO-3671 by materializing enum values as their generated CLR types in SpecificDatumReader.

Previously, enum values were returned as boxed integer ordinals, losing branch identity in unions such as ["int", "Enum1", "Enum2"].

Resolve the reader enum type through ObjectCreator and create the CLR enum value from the resolved reader ordinal. Existing symbol translation, default handling, generic-reader behavior, and public APIs remain unchanged.

Callers relying on boxed integer enum results must now handle the generated CLR enum types.

Verifying this change

Added 21 regression cases using generated CLR types, covering:

• Ordinary and nullable enum fields.
• Integer/enum and multiple-enum unions, including ["int", "Enum1", "Enum2"] and ["null", "int", "Enum1"].
• Runtime type preservation and byte-preserving reserialization.
• Reordered symbols, reader enum defaults, and missing-field defaults.
• Unchanged generic enum materialization.

Validation on Windows:

• Release build passed with zero warnings or errors.
• C# test suite passed across .NET 8, 9, and 10, including interop and package-content tests.

Documentation

• Does this pull request introduce a new feature? No; this is a bug fix.
• If yes, how is the feature documented? Not applicable.