[BUG][Typescript-fetch] Nested list with union types breaks modelPropertyNaming between Python backend and NodeJS frontend

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• 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?

Description

When generating

openapi-generator version

7.6.0.

I tried with the latest version (7.7.0), but that caused null problems during maps. That may be opened as a separate bug.

OpenAPI declaration file content or url

--> https://gist.github.com/cajmorgan/5e1f138562afa39701607c68b124a1a8

Unfortunately, I can't provide the complete OpenAPI declaration file, but here is the details for the problematic model. Maybe this information is not enough.

Generation Details

The openapi schema is generated with FastAPI, and I have validated it and all seems fine. The problem is most likely due to a bug in typescript-fetch.

The models are generated using npm:

    "generate-api": "npx openapi-generator-cli generate -i http://localhost:3001/openapi.json  -g typescript-fetch -o api",

Steps to reproduce

I've seen multiple PRs with similar errors, but nothing exactly like this, I can't provide all details as the code is not open source.

Briefly:

1. run the generate-api command, all generates fine
2. Fetch the api where the problematic model resides
3. from snake_case -> camelCase conversion is ignore on the union[NestedModel1, NestedModel2] in the nested list, therefore the .some_attribute is accessible but the typed .someAttribute is not.
4. When a request is made to the server again, with the same problematic model in the requestParams, it doesn't get converted back to snake_case, so the backend receives the model with camelCase attributes, which then produces an error.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/2976

Suggest a fix

There is something going on with the Union[] python type and typescript-fetch that produces these ambiguous behaviors. I'd suggest to handle Union types differently so that they don't get compressed into the same model, which currently happens. List[List[Union[Model1, Model2]]] -> ModelStrangeNamingInnerInner, produces one single model in the frontend. Something here then breaks the snake_case -> camelCase -> snake_case automatic conversion

Start with the linked gist and reproduce using the typescript-fetch generator command shown in the issue. Inspect the generated nested-list union model and its request/response naming conversions; done means snake_case to camelCase and back works for the affected model without collapsing the union incorrectly.