fix(utils): Fix file backup logic in WriteToFile and enhance test coverage

[sakura-3]

What is the purpose of the change

In the internal/utils/files.go package, the WriteToFile function failed to properly backup existing file contents when overwriting files. Specifically:

When writing to an existing path (e.g., aaa/exist.txt), the function only create a backup file (aaa/exist.txt.bak) before overwriting, but write nothing, leading to potential data loss.

Existing test cases lacked coverage for backup logic and related file utility functions.

Brief changelog

• fix internal/utils/files.go:WriteToFile
• add internal/utils/files_test.go
• add internal/utils/set_test.go

Verifying this change

see test file

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