[fix](sync_point) Fix heap-use-after-free in SyncPoint during program exit

[wyxxxcat]

What problem does this PR solve?

The SyncPoint singleton was being destroyed during program exit while
background bthread timers were still accessing it, causing heap-use-after-free.

Changed get_instance() to return a never-destructed heap-allocated pointer
instead of a stack-allocated static object to prevent destruction order issues.

This is a standard pattern for singletons accessed by background threads.

11:47:40       #0 0x5582a963153d in operator new(unsigned long) (/root/doris/cloud/ut_build_ASAN/test/recycler_test+0x122b53d) (BuildId: 1191affa24589f52)
11:47:40       #1 0x5582ab0aa52f in doris::SyncPoint::SyncPoint() /root/doris/common/cpp/sync_point.cpp:77:9
11:47:40       #2 0x5582ab0aa46b in doris::SyncPoint::get_instance() /root/doris/common/cpp/sync_point.cpp:73:20
11:47:40       #3 0x5582aae33ee8 in doris::cloud::memkv::Transaction::get(std::basic_string_view<char, std::char_traits<char>>, std::basic_string_view<char, std::char_traits<char>>, std::unique_ptr<doris::cloud::RangeGetIterator, std::default_delete<doris::cloud::RangeGetIterator>>*, bool, int) /root/doris/cloud/src/meta-store/mem_txn_kv.cpp:276:5
11:47:40       #4 0x5582a968f49f in doris::cloud::txn_get(doris::cloud::TxnKv*, std::basic_string_view<char, std::char_traits<char>>, std::basic_string_view<char, std::char_traits<char>>, std::unique_ptr<doris::cloud::RangeGetIterator, std::default_delete<doris::cloud::RangeGetIterator>>&) /root/doris/cloud/src/recycler/recycler.cpp:99:18
11:47:40       #5 0x5582a9657090 in doris::cloud::InstanceRecycler::scan_and_recycle(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>, std::basic_string_view<char, std::char_traits<char>>, std::function<int (std::basic_string_view<char, std::char_traits<char>>, std::basic_string_view<char, std::char_traits<char>>)>, std::function<int ()>) /root/doris/cloud/src/recycler/recycler.cpp:2976:23
11:47:40       #6 0x5582a9686153 in doris::cloud::InstanceRecycler::recycle_rowsets() /root/doris/cloud/src/recycler/recycler.cpp:2587:15
11:47:40       #7 0x5582a96a8d47 in doris::cloud::RecyclerTest_recycle_empty_Test::TestBody() /root/doris/cloud/test/recycler_test.cpp:1136:5

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

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?

[wyxxxcat]

run buildall

[doris-robot]

Cloud UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	79.26% (1796/2266)
Line Coverage	64.50% (32194/49910)
Region Coverage	65.41% (16125/24653)
Branch Coverage	55.94% (8593/15360)

[hello-stephen]

BE UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.56% (19635/37358)
Line Coverage	36.17% (183272/506660)
Region Coverage	32.45% (142081/437860)
Branch Coverage	33.43% (61674/184487)