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/other/arguments.rs

@@ -1,5 +1,5 @@
 use ruff_formatter::{write, FormatContext};
-use ruff_python_ast::{ArgOrKeyword, Arguments, Expr, StringLike};
+use ruff_python_ast::{ArgOrKeyword, Arguments, Expr, StringFlags, StringLike};
 use ruff_python_trivia::{PythonWhitespace, SimpleTokenKind, SimpleTokenizer};
 use ruff_text_size::{Ranged, TextLen, TextRange, TextSize};
 
@@ -137,6 +137,7 @@ fn is_single_argument_parenthesized(argument: &Expr, call_end: TextSize, source:
 
     false
 }
+
 /// Returns `true` if the arguments can hug directly to the enclosing parentheses in the call, as
 /// in Black's `hug_parens_with_braces_and_square_brackets` preview style behavior.
 ///
@@ -223,7 +224,13 @@ fn is_huggable_string_argument(
     arguments: &Arguments,
     context: &PyFormatContext,
 ) -> bool {
-    if string.is_implicit_concatenated() || !string.is_multiline(context.source()) {
+    if string.is_implicit_concatenated()
+        || !string.is_multiline(context)
+        || !string
+            .parts()
+            .next()
+            .is_some_and(|part| part.flags().is_triple_quoted())
+    {
         return false;
     }
 

crates/ruff_python_formatter/src/other/f_string.rs

@@ -140,6 +140,10 @@ impl FStringLayout {
         }
     }
 
+    pub(crate) const fn is_flat(self) -> bool {
+        matches!(self, FStringLayout::Flat)
+    }
+
     pub(crate) const fn is_multiline(self) -> bool {
         matches!(self, FStringLayout::Multiline)
     }