[http-client-csharp] Consolidate InputType graph walkers into a single visitor

[JoshLove-msft]

Background

While reviewing #10666 (dynamic loading of ExternalTypes), @JoshLove-msft noted that ExternalTypeReferenceResolver.CollectExternalTypes/VisitType duplicates the recursive type-graph traversal already implemented in InputModelTypeConverter.MarkModelsAsDynamicRecursive.

A third (or fourth) walker is likely needed in the future, so we should consolidate to a single canonical visitor.

Current duplication

Two callers in the repo today:

1. InputModelTypeConverter.MarkModelsAsDynamicRecursive (Microsoft.TypeSpec.Generator.Input/src/InputTypes/Serialization/InputModelTypeConverter.cs)

   • Seed: a single InputModelType (during deserialization, before the library is fully built)
   • Side effect: sets modelType.IsDynamicModel = true
   • Special rule: walks BaseModel only when the base has a DiscriminatorProperty / DiscriminatorValue

2. ExternalTypeReferenceResolver.CollectExternalTypes/VisitType (Microsoft.TypeSpec.Generator/src/Utilities/ExternalTypeReferenceResolver.cs)

   • Seed: the whole InputLibrary (Models + Enums + Constants + every client method parameter & response body)
   • Side effect: collects InputExternalTypeMetadata entries with non-null Package into a dictionary keyed by (package, version, identity)

Both walk: InputModelType → derived/discriminated subtypes → BaseModel → properties; and InputArrayType / InputDictionaryType / InputNullableType / InputUnionType value/variant types. Cycle detection via identity-based HashSet<InputType>.

Proposed shape

A static class InputTypeWalker (or instance InputTypeVisitor) in Microsoft.TypeSpec.Generator.Input that takes:

• A seed (InputType, IEnumerable<InputType>, InputNamespace, or InputLibrary overloads)
• A visit callback that returns a VisitAction enum: Continue / SkipChildren / Stop

public enum VisitAction { Continue, SkipChildren, Stop }

public static class InputTypeWalker
{
    public static void Walk(InputType root, Func<InputType, VisitAction> visit);
    public static void Walk(InputNamespace ns, Func<InputType, VisitAction> visit);
    public static void Walk(InputLibrary library, Func<InputType, VisitAction> visit);
}

Why a visitor-with-callback instead of IEnumerable<InputType> on InputLibrary (the original suggestion):

• Each caller needs different control (side effects, pruning, cycle handling).
• IEnumerable would force callers to either accept the full enumeration cost or re-implement filtering.
• A callback also lets the special-case rules (e.g. MarkModelsAsDynamicRecursive's base-discriminator walk) live inside a single owner-decision point rather than being scattered across the visitor and the consumer.

Migration

1. Land InputTypeWalker with a comprehensive set of unit tests (cycles, every container type, every entry-point overload).
2. Migrate MarkModelsAsDynamicRecursive — must preserve the base-discriminator special rule and produce the same set of dynamically-marked models on the existing test corpus.
3. Migrate ExternalTypeReferenceResolver.CollectExternalTypes — must produce the same metadata set on the existing tests.
4. Delete the duplicated traversal code.

Acceptance criteria

• One canonical type-graph walker in Microsoft.TypeSpec.Generator.Input.
• All existing MarkModelsAsDynamicRecursive and ExternalTypeReferenceResolver tests pass unchanged.
• New unit tests cover the walker directly (every node type, cycles, prune/stop semantics).

cc @JoshLove-msft (per discussion in #10666 (review thread r3231221827))

[github-actions]

🎯 Issue Triage

This is a refactoring/feature request to consolidate duplicated InputType graph traversal logic in the C# client emitter into a single canonical InputTypeWalker visitor class.

Category: Feature Request
Area: emitter:client:csharp

🔍 Analysis

• Affected components: Microsoft.TypeSpec.Generator.Input — specifically InputModelTypeConverter and ExternalTypeReferenceResolver
• Two independent type-graph walkers currently exist with overlapping traversal logic (both handle InputModelType, InputArrayType, InputDictionaryType, InputNullableType, InputUnionType, cycle detection via HashSet)
• Proposed solution is a static class InputTypeWalker with callback-based visitation and a VisitAction enum (Continue/SkipChildren/Stop)
• The issue has a clear migration plan and acceptance criteria, making it well-scoped for implementation

📝 Suggested next steps

• Implement InputTypeWalker in Microsoft.TypeSpec.Generator.Input with unit tests covering all node types, cycles, and control actions
• Migrate MarkModelsAsDynamicRecursive to use InputTypeWalker (preserve base-discriminator special rule)
• Migrate ExternalTypeReferenceResolver.CollectExternalTypes to use InputTypeWalker
• Delete the now-duplicate traversal code
• Verify all existing tests pass unchanged

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