fix(pipeline): decode GIAS extracts as Windows-1252

GIAS publishes its CSVs in Windows-1252 and sends no charset. The tap read
resp.text, so requests guessed the codec, and the encoding="latin-1" passed
to read_csv did nothing on already-decoded text. On 3 Oct 2026 the guess was
windows-1250, and "St Thomas à Becket" (138950, 149557) was stored as
"St Thomas ŕ Becket". A different guess on another day would garble other
accented names.

Both streams now decode the downloaded bytes themselves (gias_csv.py). A byte
Windows-1252 leaves undefined becomes U+FFFD with a logged warning instead of
failing the load, so one odd name cannot stop the daily refresh. None of the
nine extracts checked (1 Jul to 3 Oct 2026) contains such a byte.

Checked by running the tap on the real 3 Oct extract with .text forced to
windows-1250: all 52,586 rows decode, with no "ŕ" and no replacement
characters.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pipeline/tests/test_gias_encoding.py

@@ -0,0 +1,66 @@
+"""GIAS publishes its extracts in Windows-1252 and declares no charset.
+
+The tap used to hand pandas `resp.text`, so requests guessed the codec.
+On 3 Oct 2026 it guessed windows-1250, and "St Thomas à Becket" was stored
+as "St Thomas ŕ Becket". The `encoding=` passed to read_csv did nothing,
+because the text was already decoded.
+"""
+import importlib.util
+import logging
+from pathlib import Path
+
+import pytest
+
+MODULE = (Path(__file__).resolve().parents[1] / 'plugins' / 'extractors' / 'tap-uk-gias'
+          / 'tap_uk_gias' / 'gias_csv.py')
+
+
+@pytest.fixture
+def gias_csv():
+    spec = importlib.util.spec_from_file_location('gias_csv', MODULE)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+# Byte for byte as GIAS writes it: 0xE0 à, 0x92 ’, 0xE9 é, 0xB0 °, 0xE7 ç.
+EXTRACT = (
+    b'"URN","EstablishmentName","HeadLastName"\r\n'
+    b'"138950","St Thomas \xe0 Becket Catholic Secondary School","Smith"\r\n'
+    b'"100000","The Dean and Chapter of St Paul\x92s Cathedral","Pr\xe9vert"\r\n'
+    b'"140677","North Star 180\xb0","Fran\xe7ois"\r\n'
+    b'"100001","No head recorded",""\r\n'
+)
+
+
+def test_names_decode_as_windows_1252(gias_csv):
+    df = gias_csv.read_gias_csv(EXTRACT)
+    assert list(df['EstablishmentName']) == [
+        'St Thomas à Becket Catholic Secondary School',
+        'The Dean and Chapter of St Paul’s Cathedral',
+        'North Star 180°',
+        'No head recorded',
+    ]
+    assert list(df['HeadLastName']) == ['Smith', 'Prévert', 'François', '']
+
+
+def test_the_codec_requests_guessed_is_not_used(gias_csv):
+    # What the tap stored on 3 Oct: the same bytes read as windows-1250.
+    assert 'ŕ' in EXTRACT.decode('cp1250')
+    names = ' '.join(gias_csv.read_gias_csv(EXTRACT)['EstablishmentName'])
+    assert 'ŕ' not in names
+
+
+def test_values_stay_strings(gias_csv):
+    df = gias_csv.read_gias_csv(EXTRACT)
+    assert df.loc[0, 'URN'] == '138950'
+
+
+def test_a_byte_windows_1252_leaves_undefined_does_not_stop_the_load(gias_csv, caplog):
+    # 0x81 has no Windows-1252 character. One odd name must not block the daily
+    # refresh of every school, but it must be visible in the log.
+    extract = b'"URN","EstablishmentName"\r\n"100002","Odd \x81 Name"\r\n'
+    with caplog.at_level(logging.WARNING):
+        df = gias_csv.read_gias_csv(extract, logger=logging.getLogger('gias'))
+    assert df.loc[0, 'EstablishmentName'] == 'Odd � Name'
+    assert 'could not be decoded' in caplog.text