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/features/providers/components/ProviderFormDialog.vue

@@ -155,17 +155,6 @@
               </SelectContent>
             </Select>
           </div>
-          <div class="space-y-1.5">
-            <Label>{{ legacyT('最大重试次数') }}</Label>
-            <Input
-              :model-value="form.max_retries ?? ''"
-              type="number"
-              min="0"
-              max="999"
-              :placeholder="legacyT('默认 2')"
-              @update:model-value="(v) => form.max_retries = parseNumberInput(v)"
-            />
-          </div>
         </div>
 
         <!-- 超时配置 -->