Bump to version 4.2.7

[msrathore-db]

Summary

Bump version to 4.2.7 (in pyproject.toml and src/databricks/sql/__init__.py) and add the 4.2.7 section to CHANGELOG.md.

This refreshes the release-prep work in #797, which was cut from an older main and is missing 5 commits that have since landed (#796, #803, #814, #817, #819). This branch is rebased on current main with a complete changelog. Recommend closing #797 in favor of this PR.

Notable changes since 4.2.6

• Add experimental Rust kernel backend behind the use_kernel=True flag — routes through the Rust kernel via PyO3 with OAuth (M2M/U2M), TLS/mTLS, Geometry, complex types, parameter binding, and Thrift-backend surface parity (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, fix(backend/kernel): comparator parity — async retention + intervals_as_string + precision/scale + named params + utils.exc fix #803, feat(kernel): wire OAuth (M2M/U2M), TLS/mTLS, and Geometry on use_kernel path #819)
• Extract SPOG org-id from cluster http_path for non-Thrift requests (Extract SPOG org-id from cluster http_path for non-Thrift requests #817)
• Remove empty chunks in CloudFetch concatenation (Remove Empty Chunks in Cloudfetch concatenation #814)
• Add _retry_server_directed_only mode for Retry-After header compliance (Add _retry_server_directed_only mode for Retry-After header compliance #756)
• Bump thrift to 0.23.0 (Bump thrift to 0.23.0 #796)
• 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 plan

• pytest tests/unit/ passes locally (809 passed, 4 skipped)
• Multi-DBR validation (universe peco/correctness/python) — pending
• CI green

This pull request was AI-assisted by Isaac.