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.

agent/tools/utils/__init__.py

@@ -15,6 +15,7 @@ from .diff import (
     normalize_to_lf,
     restore_line_endings,
     normalize_for_fuzzy_match,
+    count_matches,
     fuzzy_find_text,
     generate_diff_string,
     FuzzyMatchResult
@@ -39,6 +40,7 @@ __all__ = [
     'normalize_to_lf',
     'restore_line_endings',
     'normalize_for_fuzzy_match',
+    'count_matches',
     'fuzzy_find_text',
     'generate_diff_string',
     'FuzzyMatchResult',