Bump to version 4.3.0

[vikrantpuppala]

Summary

Promote 4.3.0rc1 to the final 4.3.0 release. Bumps pyproject.toml + __init__.py to 4.3.0 and renames the 4.3.0rc1 CHANGELOG section to 4.3.0 (with a more holistic description of the kernel backend). No code changes since 4.3.0rc1 (8dd350dc) — this is a version-only promotion.

Highlights

• New: optional Rust kernel backend (use_kernel=True) via the databricks-sql-connector[kernel] extra — a native databricks-sql-kernel (Rust/PyO3) client over the SEA HTTP transport (CloudFetch + inline Arrow), Python >= 3.10, PAT/OAuth-M2M/U2M. Default backend stays Thrift; opt in per connection.
• Behavior aligned with Thrift (cursor state, metadata, rowcount, async state, cancel, staging, error context, logging).
• Revert the thrift 0.23.0 bump that broke install on DBR LTS (ES-1960554).

Test plan

• pytest tests/unit -m "not realkernel" passes locally (864 passed)
• Multi-DBR validated on 4.3.0rc1 (3/3) — 4.3.0 is identical code, so that signal carries over
• CI green

This is the final 4.3.0 — unpinned pip install databricks-sql-connector will resolve to it.

This pull request was AI-assisted by Isaac.