branch-4.0: [fix](filecache) avoid recursive empty dir cleanup

[liaoxin01]

Pick #63344

[hello-stephen]

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[Copilot]

Pull request overview

This PR updates local file cache cleanup to avoid recursively deleting cache directories, using an empty-directory-only removal path instead.

Changes:

• Adds LocalFileSystem::delete_empty_directory() implemented with rmdir.
• Uses empty-directory deletion during file cache block removal.
• Adds unit coverage for deleting empty vs non-empty local directories.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
be/src/io/fs/local_file_system.h	Declares the empty-directory deletion API.
be/src/io/fs/local_file_system.cpp	Implements non-recursive empty directory removal.
be/src/io/cache/fs_file_cache_storage.cpp	Switches file cache cleanup to delete directories only when empty.
be/test/io/fs/local_file_system_test.cpp	Adds tests for empty and non-empty directory deletion behavior.

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[liaoxin01]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 86.96% (20/23) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	53.21% (19392/36441)
Line Coverage	36.33% (181043/498329)
Region Coverage	32.90% (140483/427036)
Branch Coverage	33.81% (60857/179983)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 95.65% (22/23) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.56% (25527/35673)
Line Coverage	54.36% (270382/497431)
Region Coverage	51.82% (223519/431350)
Branch Coverage	53.31% (96287/180606)