Fix discriminated union property access to use camelCase property names

[Copilot]

The discriminated union serializers were using the original PascalCase property names from TypeSpec (e.g., DocumentType) in switch statements, but the generated TypeScript interfaces use camelCase property names (e.g., documentType). This caused TypeScript compilation errors:

src/models/models.ts:134:16 - error TS2551: Property 'DocumentType' does not exist on type 'DocumentIngressUnion'. Did you mean 'documentType'?
  Property 'DocumentType' does not exist on type 'Request'.

134   switch (item.DocumentType) {
                   ~~~~~~~~~~~~

Consider this TypeSpec definition:

@discriminator("DocumentType")
model DocumentIngress {
  DocumentType: DocumentType;
  DocumentStreamIds?: string[];
  Properties?: string[];
}

model Request extends DocumentIngress {
  DocumentType: DocumentType.Request;
  Name?: string;
  Url?: string;
}

Previously, the generated serializer would incorrectly use:

switch (item.DocumentType) { // Error: DocumentType doesn't exist on interface

Now it correctly uses:

switch (item.documentType) { // Correct: uses camelCase property name

Changes made:

• Updated buildSerializerFunction.ts to use normalizeName(discriminatorProperty.name, NameType.Property) in switch statements
• Updated buildDeserializerFunction.ts to use normalizeName(discriminatorProperty.name, NameType.Property) in switch statements
• Added comprehensive test case demonstrating the fix for PascalCase discriminator properties
• All existing tests continue to pass

The fix ensures that discriminated union serializers and deserializers correctly access discriminator properties using the normalized camelCase names that match the generated TypeScript interface properties.

Fixes #3435.

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[v-jiaodi]

@copilot update return value void to DocumentIngress in order to also cover Deserializer case.

[Copilot]

Updated the return type from void to DocumentIngress and added all the required deserializer functions. This now provides comprehensive test coverage for both serializers and deserializers with discriminated unions. Changes committed in 738008e.