feat(declarative): Add backoff jitter support (#1038)

Adds optional `jitter_range_in_seconds` support for declarative constant and exponential backoff strategies.

This lets connector devs add a small numeric jitter value in the manifest so retry timing is spread out instead of every worker retrying at the same time. The value is validated as non-negative, applied above the normal backoff time, and covered with schema, factory, and backoff strategy tests.

Co-authored-by: Zane Hyatt ZaneHyattAB
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>