[improvement](be) Default async cache writes to drop oldest

### What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: The locked FIFO async file-cache writer supports both reject_new and drop_oldest, but the initial default retained reject_new. Make drop_oldest the production and service-option default so newly downloaded blocks remain available to adjacent reads under queue saturation, while keeping reject_new available through the dynamic configuration for rollback and A/B comparison.

### Release note

`async_file_cache_write_queue_full_policy` now defaults to `drop_oldest`. Set it to `reject_new` to restore the previous admission behavior.

### Check List (For Author)

- Test: No need to test (default-selection-only change; both policy implementations and runtime switching are covered by existing unit tests)
    - BE clang-format and check-format
- Behavior changed: Yes. Full async write queues now replace the oldest queued task by default.
- Does this need documentation: No

Author: bobhan1

be/src/common/config.cpp

@@ -1286,7 +1286,7 @@ DEFINE_mInt64(async_file_cache_write_max_pending_tasks_per_disk, "256");
 DEFINE_mInt32(async_file_cache_write_batch_size, "16");
 DEFINE_mInt64(async_file_cache_write_watchdog_warn_secs, "30");
 DEFINE_mInt64(async_file_cache_write_watchdog_drop_secs, "120");
-DEFINE_mString(async_file_cache_write_queue_full_policy, "reject_new");
+DEFINE_mString(async_file_cache_write_queue_full_policy, "drop_oldest");
 DEFINE_mBool(enable_async_file_cache_write_inflight_write_buffer_index, "true");
 DEFINE_Int32(async_file_cache_write_inflight_write_buffer_index_shard_count, "64");
 DEFINE_Validator(async_file_cache_write_workers_per_disk,

be/src/io/cache/async_cache_write_service.h

@@ -107,7 +107,7 @@ struct AsyncCacheWriteServiceOptions {
     size_t batch_size {1};
     int64_t watchdog_warn_secs {30};
     int64_t watchdog_drop_secs {120};
-    AsyncCacheWriteQueueFullPolicy queue_full_policy {AsyncCacheWriteQueueFullPolicy::REJECT_NEW};
+    AsyncCacheWriteQueueFullPolicy queue_full_policy {AsyncCacheWriteQueueFullPolicy::DROP_OLDEST};
 };
 
 /// Owns the bounded async-write queue and workers for one BlockFileCache (one cache disk).

be/src/io/tools/async_file_cache_write_microbench.cpp

@@ -499,7 +499,7 @@ class BenchmarkEnvironment {
         config::async_file_cache_write_max_pending_tasks_per_disk =
                 static_cast<int64_t>(std::max(FLAGS_reader_operations, FLAGS_service_operations));
         config::async_file_cache_write_batch_size = 16;
-        config::async_file_cache_write_queue_full_policy = "reject_new";
+        config::async_file_cache_write_queue_full_policy = "drop_oldest";
 
         DORIS_CHECK(ExecEnv::GetInstance()->file_cache_factory() == nullptr);
         ExecEnv::GetInstance()->set_file_cache_open_fd_cache(std::make_unique<FDCache>());

be/test/io/cache/async_cache_write_service_test.cpp

@@ -122,6 +122,7 @@ TEST_F(AsyncCacheWriteServiceTest, TaskWritesDownloadedBlockAndCleansInflightEnt
     auto* index = cache->inflight_write_buffer_index();
     ASSERT_NE(service, nullptr);
     ASSERT_NE(index, nullptr);
+    EXPECT_EQ(service->options().queue_full_policy, AsyncCacheWriteQueueFullPolicy::DROP_OLDEST);
     const uint64_t baseline_submitted = service->_submitted_metric->get_value();
     const uint64_t baseline_submitted_bytes = service->_submitted_bytes_metric->get_value();
     const uint64_t baseline_finished = service->_finished_metric->get_value();