fix: fetch app docs_url

[chipytux]

Pull Request Description

Title: Fetch docs_url dynamically from FastAPI app

Description:

This pull request introduces a change to the fastapi-cli library that fetches the docs_url value dynamically from the FastAPI application instead of using a hardcoded value.

Implementation Details:

The change involves modifying the _run function to fetch the docs_url value from the FastAPI application using the app.openapi_url attribute. This ensures that the library always uses the URL specified by the application itself.

Testing:

The updated function has been thoroughly tested to ensure it correctly retrieves the docs_url value from the FastAPI application and show the appropriate URL.

 ╭────────── FastAPI CLI - Development mode ───────────╮                        
 │                                                     │                        
 │  Serving at: http://127.0.0.1:9901                  │                        
 │                                                     │                        
 │  API docs: http://127.0.0.1:9901/any-other-path     │                        
 │                                                     │                        
 │  Running in development mode, for production use:   │                        
 │                                                     │                        
 │  fastapi run                                        │                        
 │                                                     │                        
 ╰─────────────────────────────────────────────────────╯   

[chipytux]

Do you think this is a good approach @tiangolo ? 🙏🏻

😉 @pedroimpulcetto