Add regression test for VFS copy overwrites

[lydakis]

Summary

• add a regression test to ensure copying over an existing file overwrites the destination contents and tracks versions
• preserve node version metadata when cloning nodes so copy operations keep file versions in sync with their source

Testing

• uv run pytest tests/test_vfs.py

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[chatgpt-codex-connector]

Advance version when overwriting via copy

When _clone_node copies a file or directory it now copies the source version (sandfs/vfs.py lines 466‑469). During copy, overwriting an existing path removes the destination node and adds this clone without incrementing the version. As a result, a destination file that previously had version 1 is still reported as version 1 after being overwritten, so clients using optimistic concurrency (see tests/test_persistence.py lines 8‑13) cannot detect the change—write_file(..., expected_version=1) will succeed even though another operation replaced the file in between. Copying over an existing file should advance the destination’s version just like any other write to keep versions monotonic; mirroring the source version breaks that guarantee.

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[lydakis]

@codex

[chatgpt-codex-connector]

Summary

• Updated VirtualFileSystem.copy to proactively remove any existing destination node—either when copying directly onto a path or when placing a file inside a directory—and to advance the destination’s version so optimistic concurrency checks observe overwrites. sandfs/vfs.pyL421-L475
• Added regression coverage that ensures copy operations trigger version-mismatch errors for stale expected_version values, both for direct file targets and when copying into directories that already contain the file. tests/test_vfs.pyL1-L76

Testing

• ✅ uv run pytest tests/test_vfs.py

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