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.

frontend/src/mocks/handler.ts

@@ -977,6 +977,7 @@ const MOCK_ROUTING_GROUPS: MockRoutingGroup[] = [
         priority_mode: 'provider',
         scheduling_mode: 'cache_affinity',
         keep_priority_on_conversion: false,
+        sticky_key_attempts: 2,
       },
       model_policies: [
         {