perf: buffer accumulation in _write_query_params()

Replace the per-parameter write_value(f, param) loop in
_QueryMessage._write_query_params() with a buffer accumulation approach:
list.append + b"".join + single f.write().

This reduces the number of f.write() calls from 2*N+1 to 1, which is
significant for vector workloads with large parameters.

Also removes the redundant ExecuteMessage._write_query_params()
pass-through override to avoid extra MRO lookup per call.

Includes 14 unit tests covering normal, NULL, UNSET, empty, large vector,
and mixed parameter scenarios for both ExecuteMessage and QueryMessage.

Includes a benchmark script (benchmarks/bench_execute_write_params.py).

Author: mykaul