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/tests/snapshots/format@expression__binary.py.snap

@@ -1,7 +1,6 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/binary.py
-snapshot_kind: text
 ---
 ## Input
 ```python
@@ -425,6 +424,12 @@ if True:
                     "permissions to manage this role, or else members of this role won't receive "
                     "a notification."
                 )
+
+# This f-string should be flattened
+xxxxxxxxxxxxxxxx = f"aaaaaaaaaaaaaaaaaaaaa {
+    expression } bbbbbbbbbbbbbbbbbbbbbbbb" + (
+    yyyyyyyyyyyyyy + zzzzzzzzzzz
+)
 ```
 
 ## Output
@@ -897,4 +902,24 @@ if True:
                     "permissions to manage this role, or else members of this role won't receive "
                     "a notification."
                 )
+
+# This f-string should be flattened
+xxxxxxxxxxxxxxxx = f"aaaaaaaaaaaaaaaaaaaaa {
+    expression } bbbbbbbbbbbbbbbbbbbbbbbb" + (yyyyyyyyyyyyyy + zzzzzzzzzzz)
+```
+
+
+## Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -468,5 +468,6 @@
+                 )
+ 
+ # This f-string should be flattened
+-xxxxxxxxxxxxxxxx = f"aaaaaaaaaaaaaaaaaaaaa {
+-    expression } bbbbbbbbbbbbbbbbbbbbbbbb" + (yyyyyyyyyyyyyy + zzzzzzzzzzz)
++xxxxxxxxxxxxxxxx = f"aaaaaaaaaaaaaaaaaaaaa {expression} bbbbbbbbbbbbbbbbbbbbbbbb" + (
++    yyyyyyyyyyyyyy + zzzzzzzzzzz
++)
 ```