fix: Remove dead if sequence is None check and fix impossible test mocks

[Devguru-codes]

get_sequence() in factory.py never returns None — all code paths either return a valid sequence object or raise ValueError. This means:

1. The if sequence is None guard in main.py:34 is dead code that can never execute.
2. Two tests mock get_sequence to return None, creating impossible test scenarios that don't reflect actual behavior.

Fix

1. Removed the dead if sequence is None check from main.py.
2. Updated test mocks to use side_effect=ValueError(...) instead of return_value=None to test real failure behavior.

Files Changed

• package/src/pyaslreport/main.py
• package/src/pyaslreport/tests/test_package.py

# main.py
     sequence = get_sequence(modality, dicom_header)
     
-    if sequence is None:
-        raise ValueError(f"No matching sequence found for modality '{modality}' with the provided DICOM header")
-    
     return sequence.extract_bids_metadata()

# test_package.py
-         patch("pyaslreport.main.get_sequence", return_value=None):
+         patch("pyaslreport.main.get_sequence", side_effect=ValueError("No ASL sequence class found")):
         with pytest.raises(ValueError) as exc:
             get_bids_metadata(data)
-        assert "No matching sequence found" in str(exc.value)
+        assert "No ASL sequence class found" in str(exc.value)

---

Before/After Comparison

main branch (before fix)

#	Test	Status	Detail
1	get_sequence can return None	FAIL	Never returns None
2	main.py has dead if sequence is None check	FAIL	Dead code present
3	Tests mock get_sequence with return_value=None	FAIL	Impossible mock

Summary: PASSED=0 FAILED=3

issue13 branch (after fix)

#	Test	Status	Detail
1	get_sequence raises ValueError on failure (never returns None)	PASS	Confirmed correct behavior
2	main.py has dead if sequence is None check	PASS	Dead code removed
3	Tests mock get_sequence with return_value=None	PASS	Mocks now use side_effect=ValueError

Summary: PASSED=3 FAILED=0

Resolves #15