Added validation module with pytest tests for input handling

pytest.ini

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+[pytest]
+testpaths = tests

tests/test_validation.py

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+import sys
+import os
+import pytest
+
+# Add root directory to Python path
+sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
+
+from utils.validation import validate_file
+
+
+def test_valid_file(tmp_path):
+    file = tmp_path / "test.nii"
+    file.write_text("dummy")
+    assert validate_file(str(file)) is True
+
+
+def test_invalid_extension():
+    with pytest.raises(ValueError):
+        validate_file("file.txt")
+
+
+def test_missing_file():
+    with pytest.raises(FileNotFoundError):
+        validate_file("missing.nii")

utils/validation.py

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+import os
+
+def validate_file(path: str) -> bool:
+
+    # FIRST check extension
+    if not path.endswith(".nii"):
+        raise ValueError("Only .nii files allowed")
+
+    # THEN check existence
+    if not os.path.exists(path):
+        raise FileNotFoundError(f"{path} not found")
+
+    return True