Fix f-string formatting in assignment statement (#14454)

## Summary

fixes: #13813

This PR fixes a bug in the formatting assignment statement when the
value is an f-string.

This is resolved by using custom best fit layouts if the f-string is (a)
not already a flat f-string (thus, cannot be multiline) and (b) is not a
multiline string (thus, cannot be flattened). So, it is used in cases
like the following:
```py
aaaaaaaaaaaaaaaaaa = f"testeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee{
    expression}moreeeeeeeeeeeeeeeee"
```
Which is (a) `FStringLayout::Multiline` and (b) not a multiline.

There are various other examples in the PR diff along with additional
explanation and context as code comments.

## Test Plan

Add multiple test cases for various scenarios.

crates/ruff_python_formatter/src/expression/expr_compare.rs

@@ -29,7 +29,7 @@ impl NeedsParentheses for ExprCompare {
         } else if let Ok(string) = StringLike::try_from(&*self.left) {
             // Multiline strings are guaranteed to never fit, avoid adding unnecessary parentheses
             if !string.is_implicit_concatenated()
-                && string.is_multiline(context.source())
+                && string.is_multiline(context)
                 && !context.comments().has(string)
                 && self.comparators.first().is_some_and(|right| {
                     has_parentheses(right, context).is_some() && !context.comments().has(right)