[fix](be) Stop spill GC threads in workload group test teardown

[mrhhsg]

What problem does this PR solve?

Issue Number: None

Related PR: #66328

Problem Summary:

WorkloadGroupManagerTest creates and initializes a SpillFileManager for every test case. Initialization starts a background spill GC thread, but the fixture did not stop or delete the manager during teardown. Each subsequent test overwrote the global manager pointer, leaving the previous manager and its GC thread alive.

The leaked threads could continue accessing process-wide test utilities during static destruction and trigger an ASAN heap-use-after-free when the test binary exited. They could also interfere with tests that use global spill GC debug points.

This change releases workload-group-owned query resources first, then stops and joins the spill GC thread, deletes the manager, and finally removes the temporary spill directory. This matches the lifecycle used by other spill-related test fixtures.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
    • GTEST_REPEAT=10 ./run-be-ut.sh --run --filter='WorkloadGroupManagerTest.*:DebugPointsTest.AddTest:SpillFileTest.RetryPreservesDirectoryQueuedAfterPendingDrain' -j 8
  • 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. This only corrects test fixture resource teardown.
  • 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.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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[mrhhsg]

/review

[mrhhsg]

run beut

[github-actions]

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[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[hello-stephen]

BE UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	59.77% (26073/43621)
Line Coverage	44.09% (264389/599625)
Region Coverage	39.85% (210607/528465)
Branch Coverage	41.26% (96417/233680)

[hello-stephen]

skip buildall