fix(tap-uk-ofsted): fix header row detection matching 'urn' inside 'turn'

The preamble row in Ofsted CSVs contains 'turn off all filters' which
matched 'urn' in line.lower(), so header_idx was set to 0 instead of
the real header row. Use a regex that matches URN only as a CSV field.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pipeline/plugins/extractors/tap-uk-ofsted/tap_uk_ofsted/tap.py

@@ -137,7 +137,9 @@ class OfstedInspectionsStream(Stream):
             lines = text.split("\n")
             header_idx = 0
             for i, line in enumerate(lines[:20]):
-                if "URN" in line or "urn" in line.lower():
+                # Match lines where URN appears as a CSV field (start or after comma),
+                # not as a substring of words like "turn" or "return".
+                if re.search(r'(?:^|,)\s*URN\s*(?:,|$)', line):
                     header_idx = i
                     break
             df = pd.read_csv(