Preserve ValueError context when re-raising invalid type codes

Two ValueType conversion sites in tuples.py — the params-tuple
decoder and the row-header decoder — caught ValueError from
ValueType(raw) and re-raised as DecodeError with `from None`,
suppressing the original exception. That loses the underlying
Python enum's error phrasing (e.g., "X is not a valid ValueType"),
which is the only diagnostic detail beyond the byte value and index
we already include in the message. It also diverges from the rest
of the file, which chains with `from e` (struct errors) or lets the
raise propagate implicitly.

Change both sites to chain with `from e` so the cause is preserved
in traceback output and log-format exception chains. Extend the two
existing negative tests to assert __cause__ is a ValueError.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/tuples.py

@@ -185,8 +185,8 @@ def decode_params_tuple(
         raw_type = data[count_size + i]
         try:
             types.append(ValueType(raw_type))
-        except ValueError:
-            raise DecodeError(f"Invalid value type code {raw_type} at param index {i}") from None
+        except ValueError as e:
+            raise DecodeError(f"Invalid value type code {raw_type} at param index {i}") from e
     offset = padded_header_len
 
     # Read values
@@ -267,8 +267,8 @@ def decode_row_header(
         nibble = data[byte_idx] & 0x0F if i % 2 == 0 else (data[byte_idx] >> 4) & 0x0F
         try:
             types.append(ValueType(nibble))
-        except ValueError:
-            raise DecodeError(f"Invalid value type code {nibble} at column index {i}") from None
+        except ValueError as e:
+            raise DecodeError(f"Invalid value type code {nibble} at column index {i}") from e
 
     return types, header_size
 

tests/test_tuples.py

@@ -409,8 +409,11 @@ def test_decode_invalid_type_code_raises_decode_error(self) -> None:
 
         # Byte 0x00 means both nibbles are 0, which is not a valid ValueType
         data = b"\x00" * 8
-        with pytest.raises(DecodeError, match="Invalid value type"):
+        with pytest.raises(DecodeError, match="Invalid value type") as exc_info:
             decode_row_header(data, 1)
+        # Chain preserves the underlying ValueError so log backends show
+        # "the above exception was the direct cause" with the enum name.
+        assert isinstance(exc_info.value.__cause__, ValueError)
 
 
 class TestParamsTupleInvalidType:
@@ -422,8 +425,11 @@ def test_decode_invalid_type_code_raises_decode_error(self) -> None:
 
         # count=1, type=0 (invalid), padding to 8 bytes
         data = b"\x01\x00\x00\x00\x00\x00\x00\x00" + b"\x00" * 8
-        with pytest.raises(DecodeError, match="Invalid value type"):
+        with pytest.raises(DecodeError, match="Invalid value type") as exc_info:
             decode_params_tuple(data)
+        # Chain preserves the underlying ValueError so log backends show
+        # "the above exception was the direct cause" with the enum name.
+        assert isinstance(exc_info.value.__cause__, ValueError)
 
 
 class TestParamsTupleSchemaValidation: