Flask ImmutableMultiDict type cannot be accurately determined when calling to_dict

In Flask, the ImmutableMultiDict type cannot be accurately determined when calling to_dict()

```
from flask import Flask, render_template, request, redirect, url_for

def post(self):
f = request.form
param = f.to_dict()
vv = param["vv"]
pp = "/test/%s" % (vv)
os.system(pp)
```


Start with the Flask example using request.form and to_dict(), then inspect the CodeQL modeling for Flask's ImmutableMultiDict and its to_dict() return type. Confirm the reported type-resolution behavior and identify the relevant CodeQL tests; done means the type is accurately determined for this case.

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