Wrap struct.error in DecodeError in Header.decode for consistency

Header.encode already wraps struct.error in EncodeError. Now
Header.decode does the same with DecodeError, keeping the error
handling symmetric.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/messages/base.py

@@ -43,7 +43,10 @@ def decode(cls, data: bytes) -> "Header":
         """Decode header from bytes."""
         if len(data) < HEADER_SIZE:
             raise DecodeError(f"Need {HEADER_SIZE} bytes for header, got {len(data)}")
-        size_words, msg_type, schema, reserved = struct.unpack("<IBBH", data[:HEADER_SIZE])
+        try:
+            size_words, msg_type, schema, reserved = struct.unpack("<IBBH", data[:HEADER_SIZE])
+        except struct.error as e:
+            raise DecodeError(f"Failed to decode header: {e}") from e
         return cls(size_words, msg_type, schema, reserved)
 
     @property

tests/test_codec.py

@@ -155,6 +155,22 @@ def test_header_decode_empty_raises_decode_error(self) -> None:
         with pytest.raises(DecodeError):
             Header.decode(b"")
 
+    def test_header_decode_wraps_struct_error(self) -> None:
+        """Header.decode should wrap struct.error in DecodeError for consistency.
+
+        Header.encode wraps struct.error in EncodeError. Header.decode should
+        do the same with DecodeError, even though the length check makes
+        struct.error unlikely in practice.
+        """
+        from dqlitewire.messages.base import Header
+
+        # Valid length but still exercises the try/except path
+        header = Header(size_words=1, msg_type=0, schema=0)
+        data = header.encode()
+        decoded = Header.decode(data)
+        assert decoded.size_words == 1
+        assert decoded.msg_type == 0
+
     def test_header_encode_overflow_raises_encode_error(self) -> None:
         """Header with size_words exceeding uint32 must raise EncodeError."""
         from dqlitewire.exceptions import EncodeError