Fix E251 false positive inside f-strings

[dhruvmanila]

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:

print(f"{foo = }")

But, now it is not. This leads to false negatives such as:

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

[dhruvmanila]

Current dependencies on/for this PR:

• main
  • PR Fix E251 false positive inside f-strings #7894 👈

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