fix(filesystem): reject existing move destinations

[CryoThrust]

Summary

• reject move_file when the destination already exists
• avoid platform-dependent fs.rename overwrite behavior and potential data loss
• add an integration regression test verifying both source and destination remain intact

Closes #4628

Validation

• git diff --check
• Vitest execution is currently blocked on this host because the Homebrew Node 22 binary cannot load libsimdjson.29.dylib.

[CryoThrust]

Closing this PR because #4654 already covers the same move_file overwrite behavior. Keeping the upstream PR as the single implementation avoids duplicate work.