Summary

Bump version to 4.3.0rc1 and add the 4.3.0rc1 section to CHANGELOG.md. First release candidate of 4.3.0, carrying the optional [kernel] extra.

Notable changes since v4.2.7

• Add optional Rust kernel backend for use_kernel=True, installable via the new databricks-sql-connector[kernel] extra (requires Python >= 3.10). Routes connections through the native databricks-sql-kernel client, with parity work across cursor-state tracking, metadata, logging, staging fail-loud behavior, error context, and sync cancel (feat(kernel): surface kernel logs through Python logging on use_kernel #824, feat(kernel): wire CUJ-gap fixes — staging fail-loud, error context, sync cancel #825, fix(kernel): stop premature sync statement close (H4); bump KERNEL_REV to batch 2 #830, fix(kernel): server-sourced async state + cursor-state/metadata parity for use_kernel #838, build(kernel): add optional [kernel] extra for use_kernel=True #839).
• Revert the thrift 0.23.0 bump that broke installation on DBR LTS (ES-1960554) (Revert "Bump thrift to 0.23.0 (#796)" — fixes install failure on DBR LTS (ES-1960554) #840).

Test plan

• pytest tests/unit -m "not realkernel" passes locally (864 passed)
• Multi-DBR suites in universe/peco/correctness/python passed for 4.3.0rc1 (PythonSuite thrift + sea on 18.x; PythonUnityCatalogSuite thrift on 17.3.x — the 18.x UC run hit a known, non-driver test-harness env flake)
• CI green

This is a release-candidate (4.3.0rc1); unpinned pip install users continue to resolve to the latest stable. Published to PyPI via the secure-release pipeline.

This pull request was AI-assisted by Isaac.