refactor: unify 3 backoff implementations into shared helper

crates/zesdex-backend/src/app/util/backoff.rs

@@ -0,0 +1,30 @@
+//! Exponential backoff with jitter.
+//!
+//! Three use cases (subagent, provider, workflow) all share the same formula
+//! with different caps.  This module provides a single implementation.
+
+use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+/// Compute an exponential backoff with ±25% jitter.
+///
+/// `attempt` is 0-based (first retry -> attempt=0 -> base=1s,
+/// second retry -> attempt=1 -> base=2s, etc.).
+/// `max_secs` sets the cap.
+pub fn backoff_seconds(attempt: u32, max_secs: u64) -> Duration {
+    let base_secs = (2u64).pow(attempt).min(max_secs);
+    let quarter = (base_secs * 250_000_000).max(100_000_000); // 25% of base, min 100ms
+    let offset = jitter_ns(quarter);
+    // ±25%: offset in [0, quarter), so result = base - quarter/2 + offset
+    // which lies in [base - 25%, base + 25%).
+    let ns = base_secs * 1_000_000_000 + offset - quarter / 2;
+    Duration::from_nanos(ns)
+}
+
+/// Return a jitter offset in the range [0, range_ns).
+fn jitter_ns(range_ns: u64) -> u64 {
+    let nanos = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_nanos() as u64;
+    nanos % range_ns
+}