[BUG] [Python-fastapi] Regex patterns contain extra forward slashes in path params

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

When generating the specified content in the OpenAPI file, the regular expressions in the generated code incorrectly have slashes / added at the beginning and end of each regex pattern.

openapi-generator version

v7.9.0

OpenAPI declaration file content or url

Here is an example of the OpenAPI declaration file content:

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

Generation Details

Generated with jar:

java -jar openapi-generator-cli.jar generate -i openapi.yml -g python-fastapi -o openapi_server

Steps to reproduce

When generating the specified content in the OpenAPI file, the endpoint that includes the pattern looks like this:

async def find_product_by_code(
    code: str = Path(
    description="Product code to search", 
    regex=r"/^(?=.*[A-Z])(?=.*\d)[A-Z\d]{8,12}$/"),
) -> Product:
    """The user must have any permission in the application."""
    return await BaseProductApi.subclasses[0]().find_product_by_code(code)

Start with the python-fastapi generator invoked by java -jar openapi-generator-cli.jar generate -i openapi.yml -g python-fastapi -o openapi_server, using the linked OpenAPI declaration as the reproduction input. Compare the generated Path regex with the pattern in the issue; done means the generated pattern preserves the intended expression without extra leading or trailing slashes.