Simplify source generator internals

[shibayan]

Summary

• collapse the private C# type representation into TypeUsage
• remove redundant model state, accumulator state, and one-hop helpers
• consolidate YAML extension detection without changing supported inputs

Why

Several internal values and forwarding methods duplicated information already owned by adjacent types. Keeping a single canonical representation reduces allocations and maintenance paths while preserving the generator's behavior.

Impact

There are no intended public API, generated-source, diagnostic, MSBuild, or package contract changes.

Validation

• dotnet build OpenApiWeaver.slnx --configuration Release --no-restore
• dotnet format OpenApiWeaver.slnx --verify-no-changes --verbosity detailed --no-restore
• dotnet test OpenApiWeaver.slnx --configuration Release --no-build (153 passed)
• git diff --check

[Copilot]

Pull request overview

This PR refactors internal source-generator modeling to use a single canonical C# type representation (TypeUsage), removing redundant wrappers/state and simplifying a few helper paths while preserving generator behavior.

Changes:

• Collapses CSharpTypeReference into TypeUsage and updates all call sites to construct TypeUsage directly.
• Removes redundant internal state/one-hop helpers (e.g., tag accumulator group-name storage, enum number helper indirection).
• Centralizes YAML extension detection into a single helper used by both document filtering and document reading.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/OpenApiWeaver/OpenApi/SchemaTypeResolver.cs	Switches from TypeUsage.Create(...) to direct TypeUsage construction throughout schema type resolution.
src/OpenApiWeaver/OpenApi/SchemaEnumResolver.cs	Removes redundant BuildNumberEnumMemberName passthrough and simplifies enum member naming switch.
src/OpenApiWeaver/Models/GenerationModels.cs	Refactors TypeUsage to own all type-shape/nullability logic; simplifies computed properties using TypeUsage.
src/OpenApiWeaver/Models/CSharpTypeReference.cs	Deletes the now-redundant internal type wrapper.
src/OpenApiWeaver/Generator/Transforming/ClientGenerator.Transformer.Tags.cs	Simplifies tag-group accumulation by removing stored group name and using dictionary keys consistently.
src/OpenApiWeaver/Generator/Transforming/ClientGenerator.Transformer.Responses.cs	Updates response type resolution to construct TypeUsage directly.
src/OpenApiWeaver/Generator/Transforming/ClientGenerator.Transformer.cs	Removes redundant hasDirectionalSchemaProperties from the ClientModel constructor; computes serializer options type name as before.
src/OpenApiWeaver/Generator/ClientGenerator.DocumentReader.cs	Consolidates YAML detection into IsYamlExtension(string extension) and reuses it consistently.
src/OpenApiWeaver/Generator/ClientGenerator.cs	Minor cleanup to avoid default(ReadResult) initialization while preserving control flow.
src/OpenApiWeaver/CodeGeneration/JsonSerializerOptionsEmitter.cs	Removes a redundant overload and avoids allocations by using static lambdas where appropriate.

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