Add invalid-input rejection tests for every encoding

newsfragments/41.misc.rst

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+Added tests that verify invalid alphabet input is rejected for every encoding.

tests/test_roundtrip.py

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+"""Round-trip and invalid-input rejection tests for multibase encodings."""
+
+import pytest
+
+from multibase import ENCODINGS, DecodingError, decode
+
+INVALID_SUFFIXES = [
+    ("\u00a0", "low-unicode"),
+    ("\U0001f4a8", "emoji"),
+    ("!", "punctuation"),
+    ("\u00ff", "high-latin1"),
+]
+
+
+@pytest.mark.parametrize("encoding_info", ENCODINGS, ids=lambda e: e.encoding)
+@pytest.mark.parametrize(
+    "suffix,label",
+    INVALID_SUFFIXES,
+    ids=[label for _, label in INVALID_SUFFIXES],
+)
+def test_invalid_input_rejected(encoding_info, suffix, label):
+    """Decoding invalid alphabet characters should raise DecodingError."""
+    if encoding_info.encoding == "identity":
+        pytest.skip("identity accepts all bytes")
+    if encoding_info.encoding == "base256emoji":
+        pytest.skip("base256emoji has its own alphabet; emoji suffixes are not reliably invalid")
+
+    prefix = encoding_info.code.decode("utf-8")
+    invalid_data = prefix + suffix
+
+    with pytest.raises(DecodingError):
+        decode(invalid_data)