Remove GeneratorStubs and use CodeGen config/visitor to generate types

[ShivangiReja]

Currently, GeneratorStubs.cs contains manual CodeGenType declarations, primarily for renaming or making certain types internal. Additionally, we use GeneratorStubs to modify the namespace of subclient models. Instead of maintaining these manually, we should explore using a CodeGen visitor/config to auto-generate them. This would simplify maintenance, reduce boilerplate, and ensure consistency as new types are added.

[ShivangiReja]

I tried removing GeneratorStubs for Graders, which we currently use to update the namespace for subclient models. Here are my findings:

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Using Decorators to Change the Namespace

I experimented with the @clientNamespace decorator in client.tsp to change the namespace of the generated models.

Example usage:

• For a named model:

  @@clientNamespace(Grader, "OpenAI.Graders");

• For an anonymous model :

  @@clientNamespace(GraderStringCheck.operation, "OpenAI.Graders");

Behavior:

• ✅ Works correctly for named models—the namespace is updated as expected.
• ❌ Fails for anonymous models—applying the decorator results in an error during generation:

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Updating the Base TypeSpec Files to Use Nested Namespaces

I also tried updating the base TypeSpec files (operations.tsp, models.tsp) to define nested namespaces like OpenAI.Graders.

In operations.tsp, I used:

namespace OpenAI {
  namespace Graders {
    interface Graders {
      ...
    }
  }
}

Issue:

• ❌ This causes the GraderClient to be generated as a top-level client, which breaks the intended structure.

In models.tsp, I used:

namespace OpenAI {
  namespace Graders {
    model GraderLabelModel extends Grader { ... }
    model GraderMulti extends Grader { ... }
    ...
  }
}

Behavior:

• ✅ Named models are correctly generated under OpenAI.Graders.
• ❌ Anonymous models (like GraderStringCheckOperation, GraderTextSimilarityEvaluationMetric, etc.) are still generated under the top-level OpenAI namespace. There is a tracking issue for this in TCGC - [Bug]: Inline union does not have namespace microsoft/typespec#7985

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Summary

• Using the @clientNamespace decorator works for named models but fails for anonymous ones.
• Updating the base TypeSpec with nested namespaces works partially—named models are correctly placed, but it affects client structure and doesn’t fix anonymous model placement.
• Because of these limitations, currently we still need GeneratorStubs to enforce the correct namespace.