fix(epoch-oracle): activate validators whose epoch was skipped on startup (#1996)

## Description

The configured epoch oracle was not activating validators whose
`registration_epoch + 1` had already passed when the oracle started.
When using the hybrid oracle, if the base layer jumped ahead (e.g., from
stored epoch 0 to epoch 7321), validators queued at intermediate epochs
(e.g., 7320) were silently skipped.

## Motivation

Validators that registered on the base layer prior to the oracle
starting could fail to be added to the active set, causing them to miss
participation in consensus entirely.

## Changes

- `initialize()`: validators whose activation epoch has already passed
are now queued at `Epoch(1)` as a low sentinel, so they are picked up on
the first epoch processed instead of being dropped
- `prepare_next_epoch()`: changed from removing only the exact current
epoch key to draining all entries `<= current epoch` using
`BTreeMap::split_off`, correctly handling any skipped epochs
- Changed `queued_validators` from `HashMap` to `BTreeMap` to enable the
ordered `split_off` operation

Author: sdbondi

crates/epoch_oracles/src/configured/oracle.rs

@@ -2,7 +2,7 @@
 //   SPDX-License-Identifier: BSD-3-Clause
 
 use std::{
-    collections::{HashMap, VecDeque},
+    collections::{BTreeMap, VecDeque},
     future::poll_fn,
     task::{Context, Poll},
 };
@@ -25,7 +25,7 @@ pub struct ConfiguredEpochOracle<TStore, TTicker> {
     pending_events: VecDeque<EpochEvent>,
     store: TStore,
     ticker: TTicker,
-    queued_validators: HashMap<Epoch, Vec<Validator>>,
+    queued_validators: BTreeMap<Epoch, Vec<Validator>>,
     is_initialized: bool,
     is_done: bool,
 }
@@ -46,7 +46,7 @@ impl<TStore: EpochOracleStore + Send> ConfiguredEpochOracle<TStore, RealTimeEpoc
             store,
             ticker,
             pending_events: VecDeque::new(),
-            queued_validators: HashMap::new(),
+            queued_validators: BTreeMap::new(),
             is_initialized: false,
             is_done: false,
         })
@@ -60,7 +60,7 @@ impl<TStore: EpochOracleStore + Send, TTicker: EpochTicker> ConfiguredEpochOracl
             store,
             ticker,
             pending_events: VecDeque::new(),
-            queued_validators: HashMap::new(),
+            queued_validators: BTreeMap::new(),
             is_initialized: false,
             is_done: false,
         }
@@ -73,14 +73,16 @@ impl<TStore: EpochOracleStore + Send, TTicker: EpochTicker> ConfiguredEpochOracl
             .unwrap_or_else(Epoch::zero);
 
         for vn in &self.config.validators {
-            if vn.registration_epoch <= epoch {
-                continue;
-            }
-
-            let vns = self
-                .queued_validators
-                .entry(vn.registration_epoch + Epoch(1))
-                .or_default();
+            let activation_epoch = vn.registration_epoch + Epoch(1);
+            // If the activation epoch has already passed, activate on the next epoch we process.
+            // Use Epoch(1) as a sentinel so they are picked up by prepare_next_epoch's <= check.
+            let queue_epoch = if activation_epoch <= epoch {
+                Epoch(1)
+            } else {
+                activation_epoch
+            };
+
+            let vns = self.queued_validators.entry(queue_epoch).or_default();
             vns.push(vn.clone());
         }
 
@@ -132,10 +134,15 @@ impl<TStore: EpochOracleStore + Send, TTicker: EpochTicker> ConfiguredEpochOracl
                 }))
         }
 
-        if let Some(vns) = self.queued_validators.remove(&next_epoch) {
+        // Activate all validators queued at or before next_epoch (handles skipped epochs).
+        // split_off returns everything > next_epoch, leaving everything <= next_epoch in place.
+        let remaining = self.queued_validators.split_off(&(next_epoch + Epoch(1)));
+        let due = std::mem::replace(&mut self.queued_validators, remaining);
+
+        for (epoch, vns) in due {
             debug!(
                 target: LOG_TARGET,
-                "☘️ {} VNS activated for epoch {next_epoch}",
+                "☘️ {} VNS activated for epoch {epoch} (current: {next_epoch})",
                 vns.len()
             );
             self.pending_events