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_ast/src/nodes.rs

@@ -1273,6 +1273,15 @@ impl FStringValue {
         matches!(self.inner, FStringValueInner::Concatenated(_))
     }
 
+    /// Returns the single [`FString`] if the f-string isn't implicitly concatenated, [`None`]
+    /// otherwise.
+    pub fn as_single(&self) -> Option<&FString> {
+        match &self.inner {
+            FStringValueInner::Single(FStringPart::FString(fstring)) => Some(fstring),
+            _ => None,
+        }
+    }
+
     /// Returns a slice of all the [`FStringPart`]s contained in this value.
     pub fn as_slice(&self) -> &[FStringPart] {
         match &self.inner {