[fix](filecache) reject oversized block size in add_cell

[deardeng]

When directory_entry::file_size(ec) fails, it returns (uintmax_t)-1. load_cache_info_into_memory did not check ec, so UINT64_MAX flowed into add_cell, which only warned and then registered the cell — corrupting _files, LRUQueue::cache_size, _cur_cache_size, and the LRU dump (survives restart).

• fs_file_cache_storage: check ec after file_size, skip on error
• add_cell: turn the >1GiB check into a hard reject (return nullptr)
• UT: add_cell_rejects_oversized_size

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
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[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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[deardeng]

run buildall

[deardeng]

run buildall

[gavinchou]

what is this for?

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[deardeng]

run beut

[hello-stephen]

BE UT Coverage Report

Increment line coverage 28.57% (2/7) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	53.34% (20417/38274)
Line Coverage	36.90% (192455/521570)
Region Coverage	33.23% (149789/450789)
Branch Coverage	34.34% (65516/190778)