feat(driver): expose transaction methods

[sundy-li]

Summary

• expose begin, commit, and rollback on the public Rust Connection API
• add protocol-neutral IConnection defaults that reuse the existing exec path for REST and FlightSQL
• propagate transaction execution errors without swallowing them
• add mock-based tests for statement dispatch and error propagation

Related

Reimplements the useful parts of closed PR #360.

Verification

• cargo fmt --all -- --check
• cargo test -p databend-driver --lib
• cargo test -p databend-driver --all-features --lib
• cargo check -p databend-driver --all-features
• cargo clippy -p databend-driver --all-targets --all-features -- -D warnings
• git diff --check

Integration tests requiring a running Databend server were not run.

[sundy-li]

Closing this PR: Databend currently does not support BEGIN, COMMIT, or ROLLBACK transaction statements, so exposing these methods in the driver would advertise unsupported behavior and is not mergeable at this time. The implementation should not be merged until server-side transaction support exists.