Formatter: Better f-string dummy (#5730)

## Summary

The previous dummy was causing instabilities since it turned a string
into a variable.

E.g.
```python
            script_header_dict[
                "slurm_partition_line"
            ] = f"#SBATCH --partition {resources.queue_name}"
```
has an instability as
```python
-            script_header_dict["slurm_partition_line"] = (
-                NOT_YET_IMPLEMENTED_ExprJoinedStr
-            )
+            script_header_dict[
+                "slurm_partition_line"
+            ] = NOT_YET_IMPLEMENTED_ExprJoinedStr
```

## Test Plan

The instability is gone, otherwise it's still a dummy

crates/ruff_python_formatter/src/expression/expr_joined_str.rs

@@ -1,6 +1,6 @@
 use crate::context::PyFormatContext;
 use crate::expression::parentheses::{NeedsParentheses, OptionalParentheses};
-use crate::{not_yet_implemented, FormatNodeRule, PyFormatter};
+use crate::{not_yet_implemented_custom_text, FormatNodeRule, PyFormatter};
 use ruff_formatter::{write, Buffer, FormatResult};
 use ruff_python_ast::node::AnyNodeRef;
 use rustpython_parser::ast::ExprJoinedStr;
@@ -9,8 +9,13 @@ use rustpython_parser::ast::ExprJoinedStr;
 pub struct FormatExprJoinedStr;
 
 impl FormatNodeRule<ExprJoinedStr> for FormatExprJoinedStr {
-    fn fmt_fields(&self, item: &ExprJoinedStr, f: &mut PyFormatter) -> FormatResult<()> {
-        write!(f, [not_yet_implemented(item)])
+    fn fmt_fields(&self, _item: &ExprJoinedStr, f: &mut PyFormatter) -> FormatResult<()> {
+        write!(
+            f,
+            [not_yet_implemented_custom_text(
+                r#"f"NOT_YET_IMPLEMENTED_ExprJoinedStr""#
+            )]
+        )
     }
 }