Validate signature CLI PNG data (#1580)

* Validate signature CLI PNG data

* Keep signature CLI test imports isolated

tests/test_signature_cli_export.py

@@ -0,0 +1,40 @@
+import importlib.machinery
+import importlib.util
+import sys
+from pathlib import Path
+from types import ModuleType
+
+
+def _load_signature_cli(monkeypatch):
+    sqlalchemy_mod = ModuleType("sqlalchemy")
+    sqlalchemy_mod.text = lambda value: value
+    core_mod = ModuleType("core")
+    database_mod = ModuleType("core.database")
+    database_mod.engine = object()
+    monkeypatch.setitem(sys.modules, "sqlalchemy", sqlalchemy_mod)
+    monkeypatch.setitem(sys.modules, "core", core_mod)
+    monkeypatch.setitem(sys.modules, "core.database", database_mod)
+
+    path = Path(__file__).resolve().parent.parent / "scripts" / "odysseus-signature"
+    loader = importlib.machinery.SourceFileLoader("odysseus_signature_cli_under_test", str(path))
+    spec = importlib.util.spec_from_loader(loader.name, loader)
+    module = importlib.util.module_from_spec(spec)
+    loader.exec_module(module)
+    return module
+
+
+def test_decode_png_data_accepts_data_url(monkeypatch):
+    cli = _load_signature_cli(monkeypatch)
+
+    assert cli._decode_png_data("data:image/png;base64,aGVsbG8=") == b"hello"
+
+
+def test_decode_png_data_rejects_invalid_base64(monkeypatch):
+    cli = _load_signature_cli(monkeypatch)
+
+    try:
+        cli._decode_png_data("not valid!!!")
+    except SystemExit as exc:
+        assert exc.code == 1
+    else:
+        raise AssertionError("expected invalid base64 to exit")