feat(routing): move sticky-key retries into routing policy with lazy attempts

Replace the provider/endpoint max_retries fields as the source of same-key
retries with a routing policy setting, sticky_key_attempts (default 2). Only
the first-ranked candidate is retried on the same key; every failover
candidate gets a single attempt so failover keeps advancing instead of
retrying each fallback key.

Materialize exactly one attempt per candidate and derive same-key retries in
the attempt loop after a candidate-scoped failure, so the retry budget no
longer inflates up-front materialization and needs no upper bound. The budget
travels in the report context; retries reuse the plan with a fresh candidate
id and incremented retry index. Pool groups only retry their first key within
the retry-index stride.

Expose the setting in the routing profile editor and the set_scheduling rule
action, and drop the max_retries input from the provider form.

apps/aether-gateway/src/ai_serving/planner/candidate_ranking.rs

@@ -371,6 +371,7 @@ mod tests {
             priority_mode: aether_routing_core::RoutingSetPriorityMode::Provider,
             scheduling_mode: aether_routing_core::RoutingSchedulingMode::CacheAffinity,
             keep_priority_on_conversion: false,
+            sticky_key_attempts: aether_routing_core::DEFAULT_STICKY_KEY_ATTEMPTS,
             ranking_overlay: aether_routing_core::RankingOverlay::default(),
             mutation_plan: Default::default(),
             pool_policy_overrides: BTreeMap::new(),
@@ -406,6 +407,7 @@ mod tests {
             priority_mode: aether_routing_core::RoutingSetPriorityMode::Provider,
             scheduling_mode: aether_routing_core::RoutingSchedulingMode::FixedOrder,
             keep_priority_on_conversion: false,
+            sticky_key_attempts: aether_routing_core::DEFAULT_STICKY_KEY_ATTEMPTS,
             ranking_overlay: Default::default(),
             mutation_plan: Default::default(),
             pool_policy_overrides: Default::default(),
@@ -443,6 +445,7 @@ mod tests {
             priority_mode: aether_routing_core::RoutingSetPriorityMode::GlobalKey,
             scheduling_mode: aether_routing_core::RoutingSchedulingMode::CacheAffinity,
             keep_priority_on_conversion: false,
+            sticky_key_attempts: aether_routing_core::DEFAULT_STICKY_KEY_ATTEMPTS,
             ranking_overlay: aether_routing_core::RankingOverlay {
                 pool_priority_overrides: BTreeMap::from([("provider-1".to_string(), 4)]),
                 key_priority_overrides: BTreeMap::from([("representative-key".to_string(), 1)]),