refactor(BA-4910): Use set comprehension for container_ids deduplication

Change container_ids collection from list to set comprehension in
collect_container_stat() and collect_process_stat() to automatically
deduplicate container IDs and prevent redundant stat collection.

Benefits:
- Automatic deduplication of container IDs
- More efficient (no duplicate processing)
- Semantically clearer (expresses intent of unique IDs)
- Defensive against edge cases in kernel registry

Changes:
- src/ai/backend/agent/agent.py:1391: collect_container_stat()
- src/ai/backend/agent/agent.py:1403: collect_process_stat()

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: fregataa

src/ai/backend/agent/agent.py

@@ -1388,25 +1388,25 @@ async def collect_node_stat(self, resource_scaling_factors: Mapping[SlotName, De
     async def collect_container_stat(self, interval: float) -> None:
         if self.local_config.debug.log_stats:
             log.debug("collecting container statistics")
-        container_ids: list[ContainerId] = []
-        for kernel_obj in [*self.kernel_registry.values()]:
-            if not kernel_obj.stats_enabled or kernel_obj.container_id is None:
-                continue
-            container_ids.append(ContainerId(kernel_obj.container_id))
+        container_ids = {
+            ContainerId(kernel_obj.container_id)
+            for kernel_obj in self.kernel_registry.values()
+            if kernel_obj.stats_enabled and kernel_obj.container_id is not None
+        }
         async with asyncio.timeout(STAT_COLLECTION_TIMEOUT):
-            await self.stat_ctx.collect_container_stat(container_ids)
+            await self.stat_ctx.collect_container_stat(list(container_ids))
 
     @_observe_stat_task(stat_scope=StatScope.PROCESS)
     async def collect_process_stat(self, interval: float) -> None:
         if self.local_config.debug.log_stats:
             log.debug("collecting process statistics in container")
-        container_ids = []
-        for kernel_obj in [*self.kernel_registry.values()]:
-            if not kernel_obj.stats_enabled or kernel_obj.container_id is None:
-                continue
-            container_ids.append(ContainerId(kernel_obj.container_id))
+        container_ids = {
+            ContainerId(kernel_obj.container_id)
+            for kernel_obj in self.kernel_registry.values()
+            if kernel_obj.stats_enabled and kernel_obj.container_id is not None
+        }
         async with asyncio.timeout(STAT_COLLECTION_TIMEOUT):
-            await self.stat_ctx.collect_per_container_process_stat(container_ids)
+            await self.stat_ctx.collect_per_container_process_stat(list(container_ids))
 
     def _get_public_host(self) -> str:
         agent_config = self.local_config.agent