refactor(edit): unify fuzzy uniqueness check with the fuzzy matcher

The uniqueness guard counted occurrences via normalize_for_fuzzy_match,
while fuzzy_find_text located matches with a whitespace-flexible regex,
so the two could disagree. Extract the pattern builder as a single
source of truth (_build_fuzzy_pattern) and add count_matches, which
counts with the same exact-then-fuzzy strategy used to locate and
replace. This is the optional follow-up suggested in the review of #2942.

Adds regression tests for exact and fuzzy multi-match rejection.

tests/test_edit_tool.py

@@ -98,6 +98,41 @@ class TestEditFuzzyPreservesWhitespace(unittest.TestCase):
             "def foo():\n    x = 100\n    y = 2\n    return x + y\n",
         )
 
+    def test_exact_match_rejects_multiple_occurrences(self):
+        # Two byte-identical statements; the exact-match path applies and the
+        # uniqueness guard counts exact occurrences, so the ambiguous edit is
+        # rejected instead of silently editing only the first.
+        with open(self.path, "w", encoding="utf-8") as f:
+            f.write("a = 1\nb = 2\na = 1\n")
+        result = self.tool.execute({
+            "path": self.path,
+            "oldText": "a = 1",
+            "newText": "a = 9",
+        })
+        self.assertEqual(result.status, "error", result.result)
+        self.assertIn("occurrences", result.result)
+        # An ambiguous match must leave the file untouched.
+        self.assertEqual(self._read(), "a = 1\nb = 2\na = 1\n")
+
+    def test_fuzzy_match_rejects_multiple_occurrences(self):
+        # oldText uses loose spacing, so the exact match fails and the fuzzy
+        # path runs. The uniqueness guard now counts with the SAME regex used
+        # to match/replace, so an ambiguous fuzzy match (two hits) is rejected
+        # rather than silently editing the first one.
+        with open(self.path, "w", encoding="utf-8") as f:
+            f.write("def foo():\n    x = 1\n    y = 2\n    x = 1\n")
+        result = self.tool.execute({
+            "path": self.path,
+            "oldText": "x  =  1",
+            "newText": "x = 99",
+        })
+        self.assertEqual(result.status, "error", result.result)
+        self.assertIn("occurrences", result.result)
+        self.assertEqual(
+            self._read(),
+            "def foo():\n    x = 1\n    y = 2\n    x = 1\n",
+        )
+
 
 if __name__ == "__main__":
     unittest.main()