refactor: remove unreachable progress guard in RowsResponse.decode_body

The `if offset == prev_offset` check was added to prevent infinite loops
for zero-column results, but the zero-column handling was later extracted
into its own early-return branch. Inside the while loop, column_count >= 1
is guaranteed, and decode_row_header always advances by at least 8 bytes,
so offset always increases and the guard is unreachable dead code.

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

Author: antoineleclair

src/dqlitewire/messages/responses.py

@@ -366,8 +366,6 @@ def decode_body(
             )
 
         while offset < len(data):
-            prev_offset = offset
-
             # Read row header; markers are detected byte-by-byte inside
             result, consumed = decode_row_header(data[offset:], column_count)
             offset += consumed
@@ -404,11 +402,6 @@ def decode_body(
             if len(rows) >= max_rows:
                 raise DecodeError(f"Row count {len(rows)} reached maximum {max_rows}")
 
-            if offset == prev_offset:
-                raise DecodeError(
-                    "No progress in row decoding (possible zero-column result with malformed data)"
-                )
-
         raise DecodeError(
             f"RowsResponse body exhausted without end marker "
             f"(decoded {len(rows)} rows, consumed {offset} of {len(data)} bytes)"