Fix `E251` false positive inside f-strings (#7894)
## Summary
This PR fixes the bug where the rule `E251` was being triggered on a equal token
inside a f-string which was used in the context of debug expressions.
For example, the following was being flagged before the fix:
```python
print(f"{foo = }")
```
But, now it is not. This leads to false negatives such as:
```python
print(f"{foo(a = 1)}")
```
One solution would be to know if the opened parentheses was inside a f-string or
not. If it was then we can continue flagging until it's closed. If not, then we
should not flag it.
## Test Plan
Add new test cases and check that they don't raise any false positives.
fixes: #7882
