[BUG] Model files with specific destination folders are generated without extension

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?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Using the Python-Fastapi generator, when adding to the generator a configuration file with a custom folder for the Model files, the files in that folder are generated without extension.

openapi-generator version

v7.4.0

OpenAPI declaration file content or url

Here is an example of the OpenAPI declaration file content:

https://gist.github.com/reyescabello/6cd965708fd0b03e8eadb9f3c0c0a899

Generation Details

Configuration file:

https://gist.github.com/reyescabello/1a8fe2b9b2faa9af108947b6853f0088

Generated with docker:

docker run --rm \
-v ${PWD}:/local openapitools/openapi-generator-cli:latest generate \
-i /local/openapi.yml \
-g python-fastapi \
-o /local/api \
-c /local/config.yml

Steps to reproduce

Generating the API with the specified files creates a folder custom_model_folder with the model file product without extension, when in this case it should be a python file.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/14658

Start by running the provided Docker generation command with the linked OpenAPI declaration and configuration, then inspect custom_model_folder/product. Trace the Python-FastAPI generator path that determines model filenames; done means the generated Product model has the expected Python extension when a custom model folder is configured.