fix(alerting): populate AlertInstance.rule_snapshot so history survives rule delete

- Add withRuleSnapshot(Map) wither to AlertInstance (same pattern as other withers)
- Call snapshotRule(rule) + withRuleSnapshot in both applyResult (single-firing) and
  applyBatchFiring paths so every persisted instance carries a non-empty JSONB snapshot
- Strip null values from the Jackson-serialized map before wrapping in the immutable
  snapshot so Map.copyOf in the compact ctor does not throw NPE on nullable rule fields
- Add ruleSnapshotIsPersistedOnInstanceCreation IT: asserts name/severity/conditionKind
  appear in the rule_snapshot column after a tick fires an instance
- Add historySurvivesRuleDelete IT: fires an instance, deletes the rule, asserts
  rule_id IS NULL and rule_snapshot still contains the rule name (spec §5 guarantee)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cameleer-server-core/src/main/java/com/cameleer/server/core/alerting/AlertInstance.java

@@ -92,4 +92,11 @@ public record AlertInstance(
                 currentValue, threshold, context, title, message,
                 targetUserIds, targetGroupIds, targetRoleNames);
     }
+
+    public AlertInstance withRuleSnapshot(Map<String, Object> snapshot) {
+        return new AlertInstance(id, ruleId, snapshot, environmentId,
+                state, severity, firedAt, ackedAt, ackedBy, resolvedAt, lastNotifiedAt, silenced,
+                currentValue, threshold, context, title, message,
+                targetUserIds, targetGroupIds, targetRoleNames);
+    }
 }