Jittered exponential backoff for latest-block RPC retries

[lnbc1QWFyb24]

• Summary
  - Implement exponential backoff with full jitter when get_latest_block fails to prevent hot-looping, reduce RPC load, and cut log spam.
  Backoff resets on success and caps at 5s.
  • Changes
    • Add retry constants ERROR_RETRY_BASE_BACKOFF_MS=250 and ERROR_RETRY_MAX_BACKOFF_MS=5000 (src/agent.rs:17-19).
    • Implement jittered exponential backoff in poll_blocks:
      • On error: sleep random floor..=cap ms, double backoff_ms up to max (src/agent.rs:248-296).
      • On success: reset backoff_ms to base (src/agent.rs:255-258).
      • Log includes retry_ms for observability (src/agent.rs:292).
    • Import rand::Rng and add rand dependency (src/agent.rs:15, Cargo.toml).
    • Bump crate version to 0.1.3 (Cargo.toml).
  • Rationale
    • Avoid tight retry loops during RPC instability; align with standard backoff + jitter to minimize thundering herd behavior.
  • Behavior
    • No public API changes.
    • When RPC errors occur, retries back off between 250ms and 5s with jitter; returns to normal cadence after a successful fetch.
  • Verification
    • Induce get_latest_block errors and observe retry_ms increasing and bounded; ensure reset on success and normal block handling remains
      intact.
  • Files
    • src/agent.rs:15 — add use rand::Rng;
    • src/agent.rs:17-19 — add backoff constants
    • src/agent.rs:248-296 — backoff logic in poll_blocks
    • Cargo.toml:3 — version 0.1.3 and rand dependency
    • Cargo.lock — updated for version/deps