csv-parser: add _fingerprint helper

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_llm_csv_parser.py

@@ -29,3 +29,28 @@ def test_csv_format_template_model_columns():
     # Fingerprint is the cache key.
     assert cols["fingerprint"].unique is True
     assert cols["fingerprint"].nullable is False
+
+
+def test_fingerprint_stable_across_case_and_whitespace():
+    from app.services.llm_csv_parser import _fingerprint
+
+    a = _fingerprint(["Symbol", "Quantity", "Avg Price"])
+    b = _fingerprint(["symbol", "quantity", "avg price"])
+    c = _fingerprint(["  SYMBOL ", "Quantity", " AVG PRICE"])
+    assert a == b == c
+
+
+def test_fingerprint_differs_for_different_columns():
+    from app.services.llm_csv_parser import _fingerprint
+
+    a = _fingerprint(["Symbol", "Quantity"])
+    b = _fingerprint(["Symbol", "Quantity", "Avg Price"])
+    assert a != b
+
+
+def test_fingerprint_is_sha256_hex_64_chars():
+    from app.services.llm_csv_parser import _fingerprint
+
+    f = _fingerprint(["Symbol", "Quantity"])
+    assert len(f) == 64
+    assert all(c in "0123456789abcdef" for c in f)