fix: prevent UB stress transfer lifetime races

[YChange01]

Summary

• keep an owned TransferRequest copy in each TransferTask instead of pointing at caller-owned vectors
• defer UB successful completion publishing until the worker has finished local slice accounting
• add a regression check for TransferTask request ownership

Test Plan

• git diff --check
• cmake -S . -B /private/tmp/mooncake-build -DBUILD_UNIT_TESTS=ON -DWITH_STORE=OFF -DWITH_STORE_RUST=OFF -DWITH_TE=ON -DUSE_ETCD=OFF -DUSE_HTTP=OFF -DUSE_REDIS=OFF (blocked locally: missing yalantinglibsConfig.cmake; extern/yalantinglibs submodule is not initialized)