[BUG] [csharp-netcore] new RestClient and OAuthAuthenticator instance for every request

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• [N/A] Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

In csharp-netcore, using restsharp as the "library" option (this is the default), the ApiClient class creates an instance of a RestClient for each call to Exec() and ExecAsync(). This behavior is making it hard (or impossible) to customize the OAuth2 flow implementation for token expiration handling for example because no state can be stored (new OAuthAuthenticator instance is created for every request too). It also causes memory leaks, see related issue.

openapi-generator version

v6.2.1

OpenAPI declaration file content or url

N/A

Generation Details

Create a client using the csharp-dotnet generator, leaving the "library" option set to default (i.e. restsharp).

Steps to reproduce

Check the generated ApiClient class. On line 461, a new RestClient instance is created, the line belongs to the Exec method, which is invoked on every request.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/13961

Suggest a fix

A RestClient instance should be used as a 'Singleton', only one instance is needed. Same for OAuthAuthenticator.

Start by inspecting the generated csharp-netcore ApiClient around line 461 and tracing Exec and ExecAsync, then reproduce the default restsharp generation described in the issue. Done means the generated client reuses one RestClient and one OAuthAuthenticator across requests, preserving state for OAuth token handling and avoiding the reported leaks.