[Bug]: When Mooncake Store is running with offload enabled, offloading works correctly but a large number of offloading tasks expire after 600s, producing "Offloading task expired" warnings.

### Bug Report

When Mooncake Store is running with offload enabled, offloading works correctly but a large number of offloading tasks expire after 600s, producing "Offloading task expired" warnings.

I tried to find the cause of the problem, but I haven't been able to pinpoint it.
```
W20260624 06:46:06.259191 381518 master_service.cpp:4716] Offloading task expired for key: a4c19c15ae875bfc15ae72d0ee8e79dc0a4b44678aac35b025d7407682fee187_3_v
W20260624 06:46:06.259195 381518 master_service.cpp:4716] Offloading task expired for key: 87b17efc3475633055bdabba14c2e5604d2bfb6dccb5ae680b3ed2a2ba46e168_3_k
W20260624 06:46:06.259200 381518 master_service.cpp:4716] Offloading task expired for key: 30e3c4277dcec6c441329500295f0a512330318cd14c9213641bba673d5d5cc3_3_v
W20260624 06:46:06.259205 381518 master_service.cpp:4716] Offloading task expired for key: 0e05a4c4854a1346768028d1f9426eed9539be61de24de721f1a0378a73a722e_2_k
W20260624 06:46:06.259209 381518 master_service.cpp:4716] Offloading task expired for key: de4e52aa62013c8f66a806d5ed957ab05cbd59160e88552c23e193467b3df224_2_v
W20260624 06:46:06.259214 381518 master_service.cpp:4716] Offloading task expired for key: 6722a24e361b4d398ef7c9ced19e13badc6e6d9f8f0da8b394784241d0d366cb_0_v
W20260624 06:46:06.259218 381518 master_service.cpp:4716] Offloading task expired for key: e86accfebc712b34f1c7412dd7c552d9b450c7743947bdae668d3e24b7af6a35_1_v
W20260624 06:46:06.259222 381518 master_service.cpp:4716] Offloading task expired for key: 746e9ac98efbbbe2bd523ef0814f9bc2c1eebb0847ab879145c68a253375f8fa_0_v
W20260624 06:46:06.259227 381518 master_service.cpp:4716] Offloading task expired for key: 109fba12aedd15db207d26b1c73a766428a89e61422237c798eff54605bcf80d_2_v
W20260624 06:46:06.259232 381518 master_service.cpp:4716] Offloading task expired for key: 1d892b63241624aa4e393496a5bad9bdc7c3a24f4843453323e2f182c6535327_2_k
W20260624 06:46:06.259236 381518 master_service.cpp:4716] Offloading task expired for key: f652d924bcb15a3406c4e0282c72811431b4e00b8e91982e0201334f64ae7500_3_v
W20260624 06:46:06.275416 381518 master_service.cpp:4716] Offloading task expired for key: f86e91a717815ebd6ac761433404548dc911c37e10c5fb20134c58ab9fff5f90_2_v
W20260624 06:46:06.275429 381518 master_service.cpp:4716] Offloading task expired for key: 4ca19d0a1f69800ab41c2214a31a20659c7ae031c71efa61fb7bf8aac363fc30_2_v
W20260624 06:46:06.275436 381518 master_service.cpp:4716] Offloading task expired for key: e1eb22b8e76c7ef0338938f8315fab6956d809d73fe945815cd0dd168c9075d6_0_v
W20260624 06:46:06.275441 381518 master_service.cpp:4716] Offloading task expired for key: d5115a486c3989e4a24f3bfeb347e1975beaa823d6c14a001a1badd4fc3275b2_3_v
W20260624 06:46:06.275446 381518 master_service.cpp:4716] Offloading task expired for key: cb6a3ecd20fb91eb79a15de0d59c7a2faeec8fc57b4235bf2f2965c5d811ef9a_0_v
W20260624 06:46:06.275450 381518 master_service.cpp:4716] Offloading task expired for key: cea4a8c000f00d9789723aade34e9afec959647ed85d921bc541c6cc35469566_3_k
W20260624 06:46:06.275455 381518 master_service.cpp:4716] Offloading task expired for key: 07c754e1846bb2a92dbf4c7c162dae024dfc272cec160aed39034bc9e63e6dd1_0_v
W20260624 06:46:06.275460 381518 master_service.cpp:4716] Offloading task expired for key: 2a73466e6f95babe6fbe844eaa6c8919d86eac7059fa94ee3e50fb1055333be7_2_v
W20260624 06:46:06.275466 381518 master_service.cpp:4716] Offloading task expired for key: 636bb02ed6068232790029db294389fdc64175ade42d6e82c0c1145a7e8b77e0_0_v
W20260624 06:46:06.275470 381518 master_service.cpp:4716] Offloading task expired for key: 445ab274b56d3edb271a6473c953fd56a14b9271d6b0eb3c44c7db56b305e0d0_3_k
W20260624 06:46:06.275475 381518 master_service.cpp:4716] Offloading task expired for key: d1c79d997eef5825590847291dd102e470852f18d49f419e81ca731f121c61e3_1_k
```

**Environment**: RDMA mode, sglang with 2 Store nodes, `enable_offload=true`, latest main branch.

Mooncake master：
```
nohup \
mooncake_master \
--eviction_high_watermark_ratio=0.95 \
--eviction_ratio=0.1 \
--rpc_address ${MASTER_IP} \
--rpc_port 50051 \
--rpc_thread_num=32 \
--enable_offload=true \
--enable_http_metadata_server=true \
--http_metadata_server_port=8080 \
--enable_metric_reporting=true \
--metrics_port 9013 \
> mooncake_master.log 2>&1 &
```

Mooncake store:
```
export MOONCAKE_LOCAL_HOSTNAME="${NODE_IP}"
export MOONCAKE_TE_META_DATA_SERVER="http://${MASTER_IP}:8080/metadata"
export MOONCAKE_MASTER="${MASTER_IP}:50051"
export MOONCAKE_PROTOCOL="rdma"
export MOONCAKE_DEVICE="mlx5_0,mlx5_1"
export MOONCAKE_GLOBAL_SEGMENT_SIZE="8gb"

export MOONCAKE_OFFLOAD_FILE_STORAGE_PATH=/data1/mooncake_offload_test/
export MOONCAKE_OFFLOAD_STORAGE_BACKEND_DESCRIPTOR=bucket_storage_backend
export MOONCAKE_BUCKET_MAX_TOTAL_SIZE=$((512 * 1024 * 1024 * 1024))
export MOONCAKE_OFFLOAD_TOTAL_SIZE_LIMIT_BYTES="${MOONCAKE_BUCKET_MAX_TOTAL_SIZE}"
export MOONCAKE_BUCKET_EVICTION_POLICY=fifo
export MOONCAKE_USE_URING=true
export MOONCAKE_OFFLOAD_LOCAL_BUFFER_SIZE_BYTES=$((2048 * 1024 * 1024))
export MOONCAKE_OFFLOAD_HEARTBEAT_INTERVAL_SECONDS=2

nohup \
mooncake_client \
--master_server_address="${MOONCAKE_MASTER}" \
--host="${NODE_IP}" \
--port=10002 \
--protocol="rdma" \
--device_names="mlx5_0,mlx5_1" \
--global_segment_size="50GB" \
--enable_offload="true" \
--metadata_server="${MOONCAKE_TE_META_DATA_SERVER}" \
> mooncake_store.log 2>&1 &
```

sglang:
```
python -m sglang.launch_server \
--model /data_merged/models/Qwen3-32B-FP8 \
--port 30000 \
--tp 4 \
--enable-metrics \
--enable-hierarchical-cache \
--enable-request-time-stats-logging \
--schedule-policy lpm \
--hicache-storage-prefetch-policy timeout \
--hicache-storage-backend mooncake \
--hicache-size 100 \
--host 0.0.0.0 \
--hicache-storage-backend-extra-config '{
"master_server_address": "10.15.56.208:50051",
"local_hostname": "10.15.56.196",
"metadata_server": "http://10.15.56.208:8080/metadata",
"global_segment_size": "17179869184",
"protocol": "rdma",
"device_name": "mlx5_0,mlx5_1"
}'
```

### Before submitting...

- [ ] Ensure you searched for relevant issues and read the [documentation]

Start at master_service.cpp:4716 and inspect how offloading tasks are tracked and expired; reproduce the behavior with the provided RDMA, two-store, and sglang configuration. Done means identifying why completed or pending tasks reach the 600-second expiration and verifying that the warnings no longer occur while offloading still works.

Written by the indexing model from the issue text.