fix(tools): skip degenerate identical hunks in V4A validation

The apply phase already skips a hunk whose -/+ lines are identical
(patch_parser.py '(search_lines == replace_lines): continue'), but the
validation phase lacked the guard: such a hunk reached
fuzzy_find_and_replace, whose identical-strings error names
old_string/new_string — parameters that don't exist in patch mode — and
failed the whole atomic patch that apply would have accepted. Mirror
the apply-phase skip in validation; regression test drives a mixed
degenerate+live patch end-to-end (short text dodges the
is_already_applied >=8-char rescue).

tests/tools/test_patch_already_applied.py

@@ -140,3 +140,27 @@ class TestV4AAlreadyApplied:
         r = _patch_tool(mode="patch", patch=patch_content, task_id="t-v4a")
         assert r["success"] is True, r
         assert f.read_text() == "STATUS = 'migrated_to_v2_schema'\n"
+
+    def test_degenerate_identical_hunk_skipped_in_validation(self, workdir):
+        """A hunk whose -/+ lines are identical is a no-op: the apply phase
+        skips it, so validation must not fail the patch (previously it
+        reached fuzzy_find_and_replace, whose identical-strings error names
+        old_string/new_string — parameters that don't exist in patch mode).
+        The short text also dodges is_already_applied's >=8-char rescue."""
+        f = workdir / "degen.py"
+        f.write_text("A = 1\nB = 2\n")
+        patch_content = (
+            "*** Begin Patch\n"
+            f"*** Update File: {f}\n"
+            "-A = 1\n"
+            "+A = 1\n"
+            "@@ B @@\n"
+            "-B = 2\n"
+            "+B = 3\n"
+            "*** End Patch\n"
+        )
+        r = _patch_tool(mode="patch", patch=patch_content, task_id="t-v4a")
+        assert r["success"] is True, r
+        text = f.read_text()
+        assert "A = 1" in text  # degenerate hunk left intact
+        assert "B = 3" in text  # live hunk applied

tools/patch_parser.py

@@ -323,6 +323,14 @@ def _validate_operations(
                 replace_lines = [l.content for l in hunk.lines if l.prefix in {' ', '+'}]
                 replacement = '\n'.join(replace_lines)
 
+                if search_lines == replace_lines:
+                    # Degenerate hunk whose -/+ lines are identical: the apply
+                    # phase skips it as a no-op, so validation must not fail it
+                    # — fuzzy_find_and_replace would reject the identical
+                    # search/replacement with old_string/new_string guidance
+                    # that has no meaning in V4A patch mode.
+                    continue
+
                 new_simulated, count, _strategy, match_error = fuzzy_find_and_replace(
                     simulated, search_pattern, replacement, replace_all=False
                 )