refactor(tools): extract IDENTICAL_STRINGS_ERROR constant

The 3-sentence identical-edit message was snapshot-asserted verbatim in
two tests. House style avoids exact-string change-detector assertions;
both tests now import the constant from tools/fuzzy_match so rewording
the message can't silently break them.

tests/tools/test_fuzzy_match.py

@@ -1,6 +1,6 @@
 """Tests for the fuzzy matching module."""
 
-from tools.fuzzy_match import fuzzy_find_and_replace
+from tools.fuzzy_match import IDENTICAL_STRINGS_ERROR, fuzzy_find_and_replace
 
 
 class TestExactMatch:
@@ -29,11 +29,7 @@ class TestExactMatch:
         new, count, _, err = fuzzy_find_and_replace("abc", "abc", "abc")
         assert count == 0
         assert new == "abc"
-        assert err == (
-            "No edit was applied because old_string and new_string are identical. "
-            "Provide the existing text to replace in old_string and the changed "
-            "replacement text in new_string."
-        )
+        assert err == IDENTICAL_STRINGS_ERROR
 
     def test_multiline_exact(self):
         content = "line1\nline2\nline3"
@@ -442,12 +438,7 @@ class TestFormatNoMatchHint:
 
     def test_silent_on_identical_strings(self):
         """old_string == new_string — hint irrelevant."""
-        result = self.fmt(
-            "No edit was applied because old_string and new_string are identical. "
-            "Provide the existing text to replace in old_string and the changed "
-            "replacement text in new_string.",
-            0, "foo", "foo bar\n",
-        )
+        result = self.fmt(IDENTICAL_STRINGS_ERROR, 0, "foo", "foo bar\n")
         assert result == ""
 
     def test_silent_when_match_count_nonzero(self):

tools/fuzzy_match.py

@@ -57,6 +57,13 @@ UNICODE_MAP = {
     "\u3000": " ",  # ideographic (CJK full-width) space
 }
 
+IDENTICAL_STRINGS_ERROR = (
+    "No edit was applied because old_string and new_string are identical. "
+    "Provide the existing text to replace in old_string and the changed "
+    "replacement text in new_string."
+)
+
+
 def _unicode_normalize(text: str) -> str:
     """Normalizes Unicode characters to their standard ASCII equivalents."""
     for char, repl in UNICODE_MAP.items():
@@ -143,11 +150,7 @@ def fuzzy_find_and_replace(content: str, old_string: str, new_string: str,
         return content, 0, None, "old_string is only whitespace — provide non-blank text to match"
 
     if old_string == new_string:
-        return content, 0, None, (
-            "No edit was applied because old_string and new_string are identical. "
-            "Provide the existing text to replace in old_string and the changed "
-            "replacement text in new_string."
-        )
+        return content, 0, None, IDENTICAL_STRINGS_ERROR
 
     # Try each matching strategy in order
     strategies: List[Tuple[str, Callable]] = [