Use mv_file for single-file moves

[aryansk]

Fixes #2017

Summary

• add a default mv_file implementation that preserves the existing copy/delete behavior
• route file-only moves through mv_file so filesystems can provide an atomic override
• preserve recursive directory move behavior and add MemoryFileSystem regression coverage

Validation

• ruff format fsspec/spec.py fsspec/implementations/tests/test_memory.py
• ruff check fsspec/spec.py fsspec/implementations/tests/test_memory.py
• pytest fsspec/implementations/tests/test_memory.py -q
• pytest fsspec/tests/abstract/mv.py -q
• git diff --check

Prepared with AI assistance; I reviewed the implementation and validation output.

[martindurant]

I feel this might have been done - perhaps only for async? Or perhaps in a PR that hasn't been merged? Please check.

[aryansk]

Thanks for flagging this. I checked current master: AsyncFileSystem._mv_file already exists, and GCSFS overrides it for its atomic move path, but synchronous AbstractFileSystem.mv still calls copy followed by rm and there is no corresponding sync mv_file hook.

This draft is the sync counterpart: file-only moves go through the new default mv_file (which preserves copy/delete behavior and can be overridden by a backend), while directory moves retain the existing recursive copy/remove path. The regression test verifies that mv calls an override. If you were referring to a pending sync implementation elsewhere, please point me to it and I'll compare.