Fix C# discriminator enum naming to include package prefix

[Copilot]

C# was generating field_order oneof discriminator enums without the package prefix, inconsistent with regular C# enums, C++, and TypeScript which all include it.

Before:

// Missing "SerializationTest" prefix
public enum UnionTestMessageFieldOrderDataField : byte { ... }

After (matching C++/TypeScript):

public enum SerializationTestUnionTestMessageFieldOrderDataField : byte { ... }

Changes

• csharp_gen.py: Fixed 5 call sites of get_discriminator_enum_name() / generate_discriminator_enum() to pass the full structName ({package}{MessageName}) instead of bare msg.name. Affected locations: field declaration, deserialization, field-order dispatch, envelope methods, and file generation.

• tests/proto/test_messages.proto: Added SimplePayload (no msgid) and UnionTestMessageFieldOrder (msgid=212) to exercise the field_order discriminator path — a case previously absent from the test suite.

Original prompt

the cpp/ts generated files create an enumeration for one of messages with the auto discriminator, does the c# as well?

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