[vietddude]

Follow-up to #127, targeting its branch. Fixes from reviewing the backup provider / escalating blacklist / probe changes.

Changes

• One strike per incident. BlacklistEscalating added a strike per failed request, so several in-flight requests failing in one blip jumped the cooldown from 3m to the 30m cap. Failures during an active blacklist no longer add a strike; they only extend the expiry when the new cooldown is longer (e.g. a 403).
• Force-rotate reaches the backup. With a backup configured, a primary over ForceRotateThreshold was picked again by the primaries-first pass, so the backup was never used. It is now blacklisted for 1m (escalating, probe-gated), which sends traffic to the backup and brings the primary back on the timer. Configs without a backup keep the previous rotation behaviour.
• Total outage does not escalate. A failure right after emergency recovery reuses the current strike, so a short network or DNS outage leaves providers at their base cooldown instead of the cap.
• Stale probe results are dropped. A generation counter on the provider lets finishProbe ignore a result when the provider was recovered or re-blacklisted while the probe ran.
• Less locking. switchToPrimary takes the failover write lock only when a primary is available. Probe gating is one locked method (settleExpiredBlacklist) instead of three round-trips; RequiresProbe, ReblacklistEscalating, tryStartProbe and endProbe are gone.
• Cleanup. GetLatestBlockTimestamp uses utils.ParseHexUint64; comments trimmed to one line.

Not changed

A single Success() still resets strikes. Keeping strikes across successes would hold a rate-limited free primary out for longer and push more traffic to the paid backup.

Tests

• New: concurrent failures count as one strike, force-rotate reaches the backup and returns, total outage does not escalate, stale probe results are dropped.
• TestExecuteWithRetryProvider_FallsBackToBackup no longer sleeps the 5s retry interval.
• go test -race ./internal/rpc/ and go test ./internal/rpc/evm/ ./internal/worker/ pass.