sdk-core 0.3.9

Remove an unused telemetry environment setting from the shutdown test. Runtime behavior is unchanged from 0.3.8, including fenced failure responses for escaped worker handler errors.

Validation: formatting, all-feature Clippy, 370 Rust tests, three dependency-publication tests, and a workspace package dry run passed. Five doctests were ignored.

Python and TypeScript adopt this core version in their next SDK releases.