Avoid curly brace escape in f-string format spec (#7780)

## Summary

This PR fixes a bug in the lexer for f-string format spec where it would
consider the `{{` (double curly braces) as an escape pattern.

This is not the case as evident by the
[PEP](https://peps.python.org/pep-0701/#how-to-produce-these-new-tokens)
as well but I missed the part:

> [..]
> * **If in “format specifier mode” (see step 3), an opening brace ({)
or a closing brace (}).**
> * If not in “format specifier mode” (see step 3), an opening brace ({)
or a closing brace (}) that is not immediately followed by another
opening/closing brace.

## Test Plan

Add a test case to verify the fix and update the snapshot.

fixes: #7778

crates/ruff_python_parser/src/snapshots/ruff_python_parser__lexer__tests__fstring_with_format_spec.snap

@@ -191,11 +191,76 @@ expression: lex_source(source)
         43..44,
     ),
     (
-        FStringEnd,
+        FStringMiddle {
+            value: " ",
+            is_raw: false,
+        },
         44..45,
     ),
+    (
+        Lbrace,
+        45..46,
+    ),
+    (
+        Name {
+            name: "x",
+        },
+        46..47,
+    ),
+    (
+        Colon,
+        47..48,
+    ),
+    (
+        Lbrace,
+        48..49,
+    ),
+    (
+        Lbrace,
+        49..50,
+    ),
+    (
+        Int {
+            value: 1,
+        },
+        50..51,
+    ),
+    (
+        Rbrace,
+        51..52,
+    ),
+    (
+        Dot,
+        52..53,
+    ),
+    (
+        Name {
+            name: "pop",
+        },
+        53..56,
+    ),
+    (
+        Lpar,
+        56..57,
+    ),
+    (
+        Rpar,
+        57..58,
+    ),
+    (
+        Rbrace,
+        58..59,
+    ),
+    (
+        Rbrace,
+        59..60,
+    ),
+    (
+        FStringEnd,
+        60..61,
+    ),
     (
         Newline,
-        45..45,
+        61..61,
     ),
 ]