Improve OpenApiOperation ActualResponses and Responses performance

[lahma]

• use internal helpers and avoid allocating dictionaries

[Copilot]

Pull Request Overview

This PR improves the performance of actual OpenAPI operation response handling by introducing internal helper methods to avoid extra allocations when processing responses. Key changes include:

• Initializing a private readonly _responses field and refactoring the Responses property in OpenApiOperation.
• Replacing direct use of ActualResponses with new helper methods (GetActualResponses, HasActualResponse, GetActualResponse) across multiple files.
• Updating dependent code in OpenApiDocument, TypeScript, and CSharp models to leverage the new API.

Reviewed Changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/NSwag.Core/OpenApiOperation.cs	Refactored Responses initialization and introduced helper methods for performance improvement.
src/NSwag.Core/OpenApiDocument.cs	Updated usage to call HasActualResponse with static lambdas for better performance.
src/NSwag.CodeGeneration/Models/OperationModelBase.cs	Changed to use GetActualResponses for response model creation.
src/NSwag.CodeGeneration.TypeScript/Models/TypeScriptOperationModel.cs	Replaced ActualResponses.Any with HasActualResponse for cleaner syntax.
src/NSwag.CodeGeneration.CSharp/Models/CSharpOperationModel.cs	Updated exception type determination to use GetActualResponse.
src/NSwag.CodeGeneration.CSharp/Models/CSharpFileTemplateModel.cs	Modified collection and file response detections to use HasActualResponse.
src/NSwag.CodeGeneration.CSharp/Models/CSharpClientTemplateModel.cs	Refactored JSON converter check to use the new helper method.

Files not reviewed (2)

• src/NSwag.CodeGeneration.CSharp/NSwag.CodeGeneration.CSharp.csproj: Language not supported
• src/NSwag.Core/NSwag.Core.csproj: Language not supported

Comments suppressed due to low confidence (1)

src/NSwag.Core/OpenApiOperation.cs:191

• [nitpick] Consider adding dedicated unit tests for the new helper methods (GetActualResponses, HasActualResponse, and GetActualResponse) to ensure they work as expected in edge cases.

        // helper to avoid extra allocations