fix(csharp): escape reserved keywords in enum value declarations

[yashhzd]

Summary

Fixes #31

Enum values using C# reserved keywords (e.g. event, fixed, virtual) produce invalid C# code that fails to compile. This PR applies the existing toCSharpIdentifier method to enum value names, which prefixes reserved keywords with _ — consistent with how field names are already handled.

When an enum value name is escaped, a [System.Runtime.Serialization.EnumMember(Value = "originalName")] attribute is added to preserve the original name during JSON serialization, matching the existing pattern used for the @AcceptedValue decorator.

Changes

• lib/codegen/fromcto/csharp/csharpvisitor.js: Modified visitEnumValueDeclaration to pass enum value names through toCSharpIdentifier and add EnumMember attribute when the name is escaped.
• test/codegen/fromcto/csharp/csharpvisitor.js: Added integration test with reserved keyword enum values (event, fixed, virtual) and unit tests for both reserved and non-reserved enum value names.

Example

Before (invalid C#):

enum Status {
    Active,
    event,   // compilation error — reserved keyword
    fixed,   // compilation error — reserved keyword
}

After (valid C#):

enum Status {
    Active,
    [System.Runtime.Serialization.EnumMember(Value = "event")]
    _event,
    [System.Runtime.Serialization.EnumMember(Value = "fixed")]
    _fixed,
}

Testing

All 569 tests pass (3 new tests added). No existing tests broken.