Summary

Bump version to 4.2.7 and add the 4.2.7 section to CHANGELOG.md.

Notable changes since v4.2.6

• Add experimental Rust kernel backend (opt-in via use_kernel=True) — routes execution through the Databricks SQL kernel via PyO3, with TSparkParameter binding, Thrift backend feature parity, and _use_arrow_native_complex_types support (feat(backend/kernel): add use_kernel=True flag — route through the Rust kernel via PyO3 #787, feat(backend/kernel): wire TSparkParameter through to kernel bind_param #789, feat(backend/kernel): match Thrift backend's user-visible surface #793, feat(backend/kernel): honour _use_arrow_native_complex_types via kernel's complex_types_as_json post-processor #795)
• Add opt-in _respect_server_retry_after_header retry mode that only retries 429/503 when the server sends a Retry-After header — prevents duplicate side effects for non-idempotent operations (Add _retry_server_directed_only mode for Retry-After header compliance #756)
• Allow pandas 3.x in dependency constraints (Allow pandas 3.x in dependency constraints #768)
• Telemetry: unwrap TokenFederationProvider to report inner auth mechanism/flow (Telemetry: unwrap TokenFederationProvider to report inner auth mech/flow #781)
• Test fixes for MST metadata expectations and User-Agent env leak (Test fixes for MST metadata expectations and User-Agent env leak #788)

Test plan

• pytest tests/unit/ passes locally (751 passed, 4 skipped)
• Multi-DBR tests in universe/peco/correctness/python passing — running on https://github.com/databricks-eng/universe/pull/1967448
• CI green

This pull request was AI-assisted by Isaac.