[fix](cloud) Fix unhandled error when get_partition_boundaries

[gavinchou]

(gdb) bt
#0  doris::cloud::FdbTxnKv::get_partition_boundaries (this=0x7f11ac6e3950, boundaries=0x7f1130017f20)
    at /home/zcp/repo_center/doris_branch-4.0/doris/cloud/src/meta-store/txn_kv.cpp:212
#1  0x000055e494de9276 in doris::cloud::export_fdb_kv_ranges_details (kv=0x7f11ac6e3950) at /home/zcp/repo_center/doris_branch-4.0/doris/cloud/src/common/metric.cpp:358
#2  doris::cloud::FdbMetricExporter::export_fdb_metrics (txn_kv=0x7f11ac6e3950) at /home/zcp/repo_center/doris_branch-4.0/doris/cloud/src/common/metric.cpp:393
#3  0x000055e494dec77d in doris::cloud::FdbMetricExporter::start()::$_0::operator()() const (this=<optimized out>)
    at /home/zcp/repo_center/doris_branch-4.0/doris/cloud/src/common/metric.cpp:415
#4  std::__invoke_impl<void, doris::cloud::FdbMetricExporter::start()::$_0>(std::__invoke_other, doris::cloud::FdbMetricExporter::start()::$_0&&) (__f=...)
    at /usr/local/ldb-toolchain-v0.26/bin/../lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/invoke.h:63
#5  std::__invoke<doris::cloud::FdbMetricExporter::start()::$_0>(doris::cloud::FdbMetricExporter::start()::$_0&&) (__fn=...)
    at /usr/local/ldb-toolchain-v0.26/bin/../lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/invoke.h:98
#6  std::thread::_Invoker<std::tuple<doris::cloud::FdbMetricExporter::start()::$_0> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) (this=<optimized out>)
    at /usr/local/ldb-toolchain-v0.26/bin/../lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/std_thread.h:303
#7  std::thread::_Invoker<std::tuple<doris::cloud::FdbMetricExporter::start()::$_0> >::operator()() (this=<optimized out>)
    at /usr/local/ldb-toolchain-v0.26/bin/../lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/std_thread.h:310
#8  std::thread::_State_impl<std::thread::_Invoker<std::tuple<doris::cloud::FdbMetricExporter::start()::$_0> > >::_M_run() (this=<optimized out>)
    at /usr/local/ldb-toolchain-v0.26/bin/../lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/std_thread.h:255
#9  0x000055e495e547b0 in execute_native_thread_routine ()
#10 0x00007f11b1dfeac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#11 0x00007f11b1e90850 in __closefrom_fallback (from=-1266337808, dirfd_fallback=<optimized out>) at ../sysdeps/unix/sysv/linux/closefrom_fallback.c:45
#12 0x0000000000000000 in ?? ()

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[gavinchou]

run buildall

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[gavinchou]

run buildall

[doris-robot]

Cloud UT Coverage Report

Increment line coverage 0.00% (0/13) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	83.26% (1636/1965)
Line Coverage	67.75% (28905/42667)
Region Coverage	68.04% (14249/20942)
Branch Coverage	58.33% (7585/13004)