Support TCGC exact() client name in C# emitter

[Copilot]

TCGC 0.68.0 added exact(), which marks a @clientName value to be preserved verbatim by emitters. This change plumbs the new isExactName flag through the C# emitter and generator so that marked names bypass .ToIdentifierName() casing.

model Book {
  @clientName(Azure.ClientGenerator.Core.exact("snake_case_name"), "csharp")
  name: string;
}
// Generated C#: public string snake_case_name { get; set; }

Changes

• TypeScript emitter — Added optional isExactName?: boolean to InputModelType, InputEnumType, InputLiteralType, InputUnionType, and InputPropertyTypeBase. Populated from the corresponding SDK types in type-converter.ts, operation-converter.ts, and client-converter.ts.
• Generator.Input — Added IsExactName to the InputType and InputProperty base classes and extended the JSON converters (model, enum, literal, union, model property, and method/header/query/path/body/endpoint parameter) to deserialize the field.
• Generator providers — ModelProvider.BuildName, EnumProvider.BuildName, and PropertyProvider skip .ToIdentifierName() when IsExactName is true.
• Tests — New PropertyProviderTests cases for exact-name model and property; emitter model-type.test.ts cases verifying propagation from exact() on both a property and a model, plus a negative case.