fix(products): conservative dialect detection — canonical-distinctive columns veto Shopify (SLICE-8 review)

Addresses adversarial-review findings: a canonical file carrying a stray
Shopify-signature name (e.g. SEO Title) no longer misdetects as Shopify and
silently drops canonical Type / corrupts the weight unit (kg->g). A
canonical-distinctive column (Description/Variant Cost/Variant Weight/Google
Product Category/Variant Volume/Tax ID/Position) now vetoes detection, so
detection leans conservative (under-detection warns; over-detection corrupts).
Also closes the dual-named-column shadow (Body (HTML)+Description) and the
stale-weight-unit-on-clear case. Tests cover all three.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/test_products_dialect_shopify.py

@@ -76,3 +76,46 @@ def test_shopify_fixture_maps_exhaustively():
     ):
         assert col in parsed.unknown_columns, col
     assert "Variant Compare At Price" not in first
+
+
+# --- Detection robustness (review findings #1, #2): bias conservative ----------------
+
+
+def test_canonical_distinctive_column_vetoes_shopify_detection():
+    # A canonical file carrying a stray Shopify-signature name (`SEO Title`) stays
+    # canonical because it also has canonical-distinctive columns — no misparse,
+    # no dropped Type, no corrupted weight unit (review finding #2).
+    header = ["Handle", "Title", "Type", "Variant Weight", "Variant Weight Unit", "SEO Title"]
+    assert is_shopify_header(header) is False
+
+
+def test_dual_named_file_stays_canonical_and_warns_shopify_name():
+    # A malformed file with BOTH `Body (HTML)` and canonical `Description`: the
+    # canonical column vetoes detection, so `Body (HTML)` is honestly warned as an
+    # unknown column rather than silently shadowing `Description` (review finding #1).
+    data = (
+        b"Handle,Title,Body (HTML),Description,Variant Price\n"
+        b"mug,Moon Mug,FROM_BODY,FROM_DESC,18.00\n"
+    )
+    parsed = parse_csv(data)
+    assert parsed.dialect == "canonical"
+    assert parsed.rows[0].cells["Description"] == "FROM_DESC"
+    assert "Body (HTML)" in parsed.unknown_columns
+
+
+def test_real_shopify_export_still_detected():
+    # The conservative veto must not break a genuine Shopify export (no canonical-
+    # distinctive columns present).
+    parsed = parse_csv(_FIXTURE.read_bytes())
+    assert parsed.dialect == "shopify"
+
+
+def test_shopify_grams_clear_clears_weight_unit_together():
+    # An empty Variant Grams (a clear) clears the synthesized unit too — weight and
+    # unit move together, never a stale unit (review finding #3).
+    data = b"Handle,Title,Body (HTML),Variant Grams\nmug,Moon Mug,<p>x</p>,\n"
+    parsed = parse_csv(data)
+    assert parsed.dialect == "shopify"
+    cells = parsed.rows[0].cells
+    assert cells["Variant Weight"] == ""
+    assert cells["Variant Weight Unit"] == ""