flaky test: `RegionBlockReaderTest.OverflowColumnRowV2`

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

[1474/3344] RegionBlockReaderTest.OverflowColumnRowV2 (598 ms)
[2026/07/31 16:20:15.917 +08:00] [INFO] [LocalIndexerScheduler.cpp:62] ["Initialized LocalIndexerScheduler, pool_size=1, memory_limit_mb=0.0"] [thread_id=1]
[2026/07/31 16:20:15.917 +08:00] [INFO] [BackgroundProcessingPool.cpp:98] ["Create BackgroundProcessingPool, prefix=bg- n_threads=2"] [thread_id=1]
[2026/07/31 16:20:15.917 +08:00] [INFO] [BackgroundProcessingPool.cpp:98] ["Create BackgroundProcessingPool, prefix=bg-block- n_threads=2"] [thread_id=1]
[2026/07/31 16:20:15.917 +08:00] [INFO] [PathCapacityMetrics.cpp:94] ["Init capacity [path=/tmp-memfs/tiflash-tests/tmp/] [capacity=0.00 B]"] [thread_id=1]
[2026/07/31 16:20:15.918 +08:00] [INFO] [PageStorageImpl.cpp:54] ["PageStorageImpl start. Config{ PageStorageConfig {blob_file_limit_size: 268435456, blob_spacemap_type: 2, blob_heavy_gc_valid_rate: 0.500, blob_heavy_gc_valid_rate_raft_data: 0.050, blob_block_alignment_bytes: 0, wal_roll_size: 2097152, wal_max_persisted_log_files: 4} }"] [source=__global__.log] [thread_id=1]
[2026/07/31 16:20:15.918 +08:00] [INFO] [PageStorageImpl.cpp:54] ["PageStorageImpl start. Config{ PageStorageConfig {blob_file_limit_size: 268435456, blob_spacemap_type: 2, blob_heavy_gc_valid_rate: 0.500, blob_heavy_gc_valid_rate_raft_data: 0.050, blob_block_alignment_bytes: 0, wal_roll_size: 2097152, wal_max_persisted_log_files: 4} }"] [source=__global__.data] [thread_id=1]
[2026/07/31 16:20:15.918 +08:00] [INFO] [PageStorageImpl.cpp:54] ["PageStorageImpl start. Config{ PageStorageConfig {blob_file_limit_size: 268435456, blob_spacemap_type: 2, blob_heavy_gc_valid_rate: 0.500, blob_heavy_gc_valid_rate_raft_data: 0.050, blob_block_alignment_bytes: 0, wal_roll_size: 2097152, wal_max_persisted_log_files: 4} }"] [source=__global__.meta] [thread_id=1]
[2026/07/31 16:20:15.918 +08:00] [INFO] [BlobStore.cpp:130] ["Ignore not blob file, dir=/tmp-memfs/tiflash-tests/tmp/page/log file=wal err_msg="] [source=__global__.log] [thread_id=1]
<jemalloc>: /home/jenkins/agent/workspace/tiflash-build-common/tiflash/contrib/jemalloc/src/arena.c:138: Failed assertion: "nmalloc >= ndalloc"
Received signal Aborted
[2026/07/31 16:20:15.918 +08:00] [INFO] [PageDirectoryFactory.cpp:68] ["PageDirectory restored, max_page_id=0 max_applied_ver=0"] [source=__global__.log] [thread_id=1]
<jemalloc>: Should own 0 locks of rank >= 1: ctl(2)
[1474/3344] RegionBlockReaderTest.OverflowColumnRowV2 returned/aborted with exit code -6 (598 ms)

4. What is your TiFlash version? (Required)

https://github.com/pingcap/tiflash/pull/11009/changes/1321ccf55438cec6a69fa9df72d983a012953968

Start by locating and repeatedly running RegionBlockReaderTest.OverflowColumnRowV2, using the jemalloc assertion and abort output as the failure signature. Compare the test environment or changes referenced in pull request 11009, then identify the cause of the flake; done means the test runs reliably without the jemalloc abort.