fix(python): correct cursor rowcount semantics

[youngsofun]

Summary

• keep rowcount at -1 for result-producing and unclassified statements
• continue reporting write_rows for DML, including statements that affect zero rows
• preserve zero-progress REST pages so successful empty writes still expose ServerStats
• cache the parsed SQL AST in driver query builders and reuse it for DML classification, placeholder detection, and parameter replacement

Motivation

The target branch exposes write_rows as cursor.rowcount, but that makes a non-empty SELECT report 0. It also misses successful statements whose final response has no schema, rows, or non-zero progress, leaving zero-row DML with stats is None and rowcount == -1.

This follow-up keeps the write-statistics behavior for DML while making result-set row counts conform to PEP 249 and retaining zero-valued final statistics. SQL classification reuses the driver's existing parser result instead of parsing again in the Python binding.

Testing

• cargo fmt --all -- --check
• cargo test -p databend-client --lib
• cargo test -p databend-driver --lib
• cargo test -p databend-python --lib
• cargo check -p databend-python
• cargo clippy -p databend-client -p databend-driver -p databend-python --lib -- -D warnings
• cd bindings/python && uv run ruff format --check package/databend_driver/__init__.pyi tests/cursor/steps/binding.py
• make integration-bindings-python
• cd bindings/python && QUERY_RESULT_FORMAT=arrow .venv/bin/python -m behave tests/cursor --name 'Insert and Select'
• git diff --check

Compatibility

For a result-producing statement, cursor.rowcount remains -1. DML continues to report affected rows immediately from server statistics.

Follow-up to #791.