[BUG] [Python-Fastapi] Incorrect encoding of regex lookahead (?=...) in generated code

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 using a regular expression with a lookahead (?=...) in an OpenAPI specification, the generated code returns an incorrect encoding of the lookahead, where ?= becomes ?=.

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 OpenAPI declaration linked in the issue and reproduce generation using the shown openapi-generator CLI command with the python-fastapi generator. Compare the generated endpoint's regex in the output with the source pattern; done means the lookahead remains encoded as (?=...) rather than ?=.