Add External property to InputType to preserve original type information

[Copilot]

• Add InputExternalTypeMetadata interface to TypeScript for external type metadata
• Add external property to TypeScript InputTypeBase interface
• Create fromSdkExternalTypeInfo helper function to convert TCGC external info
• Update fromSdkType to use helper function instead of extracting inline
• Remove separate InputExternalType handling and preserve original type structure
• Create C# InputExternalTypeMetadata class
• Add External property to C# InputType class
• Create InputExternalTypeMetadataConverter for JSON serialization
• Update all type converters to handle External property
• Delete InputExternalType class and converter
• Update TypeFactory.CreateCSharpTypeCore to check External property first
• Update CreateExternalType to accept InputExternalTypeMetadata
• Update tests to validate external property is set on original types
• Add optional InputExternalTypeMetadata parameter to all applicable InputFactory methods (Union, Array, Dictionary, Model, Enum)
• Add comprehensive tests for External property deserialization (Union, Model, Array, Dictionary, Enum)
• Run all tests to ensure no regressions

Original prompt

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This section details on the original issue you should resolve

<issue_title>Include original input type information for external types</issue_title>
<issue_description>For the case of DFE in particular, we expect tsp like the following:

@doc("Expression model for DataFactory expressions")
model DfeExpressionModel {
  @doc("The type of expression")
  type: "Expression";
  @doc("The expression value")
  value: string;
}

// DataFactoryElement alternate type pattern
@alternateType({identity: "Azure.Core.Expressions.DataFactoryElement"}, "csharp")
union Dfe<T> {
  T,
  DfeExpressionModel
}

@doc("Model with DataFactoryElement properties")
model DataFactoryElementModel {
  @doc("String property with DFE pattern")
  stringProperty: Dfe<string>;

  @doc("Int property with DFE pattern")
  intProperty: Dfe<int32>;

  @doc("Bool property with DFE pattern")
  boolProperty: Dfe<boolean>;

  @doc("String array property with DFE pattern")
  stringArrayProperty: Dfe<string[]>;
}

Right now, the union is replaced in the C# emitter with an external type. We should include a property on the external type that contains the original input type that was replaced. This is needed for DFE as we need to be able to get the union variant that represents the generic argument. There may be similar scenarios for other alternate types so we should just include the original input type as a property on the constructed external type in the emitter. We will need to update the C# InputExternalType to have a corresponding Original property of type InputType.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Preserve original input type information for external types #9152

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[JoshLove-msft]

@copilot we need to delete InputExternalType

[Copilot]

@copilot we need to delete InputExternalType

Done in 8075dd5. Deleted InputExternalType class and converter. Updated TypeFactory to check the External property on types and call CreateExternalType with InputExternalTypeProperties.

[JoshLove-msft]

Contributes to Azure/azure-sdk-for-net#54347

[github-actions]

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