Python: `FlaskApp` instances do not reliably track subclasses

I was running a set of queries on the [CTFd](https://github.com/CTFd/CTFd) repository, and got suspiciously few results for endpoints. After some experimenting I found out that [they subclass `Flask`](https://github.com/CTFd/CTFd/blob/master/CTFd/__init__.py#L51C1-L91C1), which the current query for `FlaskApp::instance()` does not seem to track.

# Minimal example:
Query:
```plaintext
import python

import semmle.python.frameworks.Flask
import semmle.python.ApiGraphs

from API::Node node
where Flask::FlaskApp::instance() = node
select node
```

Python:
```py
from flask import Flask

class Sub(Flask):
def __init__(self, *args, **kwargs):
Flask.__init__(self, *args, **kwargs)

app = Sub(__name__)

@app.route("/")
def hello():
return "world"
```

CodeQL is not able to identify `Sub` as a Flask app, which means the route setup for `hello` cannot be detected as well.

Adding `.getASubclass*()` to [FlaskApp::instance()](https://github.com/github/codeql/blob/main/python/ql/lib/semmle/python/frameworks/Flask.qll#L76C1-L76C64) would probably fix this. If this can be overridden without changes to the library I am open to suggestions, my knowledge of the QL modules is not that great as of now.

從 python/ql/lib/semmle/python/frameworks/Flask.qll 中的 FlaskApp::instance() 開始，然後重現 issue 中的最小查詢和 Python 子類別範例。確認 Flask 子類別會被辨識為 app，且能偵測到 hello 的 route；如果在附近找到相關的 library coverage test，請新增或更新它。

由索引模型根據 Issue 內容生成。