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

crates/ruff_linter/resources/test/fixtures/pycodestyle/E25.py

@@ -54,3 +54,8 @@ f"{a=}"
 f"{a:=1}"
 f"{foo(a=1)}"
 f"normal {f"{a=}"} normal"
+
+# Okay as the `=` is used inside a f-string...
+print(f"{foo = }")
+# ...but then it creates false negatives for now
+print(f"{foo(a = 1)}")

crates/ruff_linter/src/rules/pycodestyle/rules/logical_lines/whitespace_around_named_parameter_equals.rs

@@ -94,6 +94,7 @@ pub(crate) fn whitespace_around_named_parameter_equals(
     context: &mut LogicalLinesContext,
 ) {
     let mut parens = 0u32;
+    let mut fstrings = 0u32;
     let mut annotated_func_arg = false;
     let mut prev_end = TextSize::default();
 
@@ -108,6 +109,8 @@ pub(crate) fn whitespace_around_named_parameter_equals(
         }
 
         match kind {
+            TokenKind::FStringStart => fstrings += 1,
+            TokenKind::FStringEnd => fstrings = fstrings.saturating_sub(1),
             TokenKind::Lpar | TokenKind::Lsqb => {
                 parens = parens.saturating_add(1);
             }
@@ -124,7 +127,7 @@ pub(crate) fn whitespace_around_named_parameter_equals(
             TokenKind::Comma if parens == 1 => {
                 annotated_func_arg = false;
             }
-            TokenKind::Equal if parens > 0 => {
+            TokenKind::Equal if parens > 0 && fstrings == 0 => {
                 if annotated_func_arg && parens == 1 {
                     let start = token.start();
                     if start == prev_end && prev_end != TextSize::new(0) {