[fix](filecache) fix crash on reset_range when clearing cache concurrently (pick#60271)

[freemandealer]

…ently (#60271)

reset_range could dereference a null cell when a block was evicted/removed while a downloader thread was finalizing a partial block. FileBlock stayed alive via refcount, but its FileBlockCell had already been erased from _files (when doing clear_cache operation), leading to SIGSEGV.

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

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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]

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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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[freemandealer]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 16.67% (1/6) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.99% (19153/36143)
Line Coverage	36.19% (178403/492984)
Region Coverage	32.78% (138246/421786)
Branch Coverage	33.79% (60045/177717)

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.