Ensure VFS state is restored after storage conflicts

[lydakis]

Summary

• add a regression test that simulates an external storage update and ensures the VFS raises a conflict while keeping its cached state intact
• restore a file's cached content and version whenever a storage adapter write fails so callers observe the previous state after the exception

Testing

• uv run pytest tests/test_adapters.py

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Do not force provider read before every write

The new rollback logic in VirtualFileSystem.write_file (sandfs/vfs.py lines 314‑315) now reads the existing file contents via node.read(self) before every write so it can restore them later. For provider‑backed files, VirtualFile.read (sandfs/nodes.py lines 57‑65) calls the provider and raises ProviderError if the provider fails. Before this change, write_file never touched the provider; callers could overwrite a file even if its provider was currently broken. After this change any failure while reading the provider aborts the write before it even attempts to persist, meaning users can no longer replace or repair a provider‑backed file whose provider errors out. To avoid this regression the rollback data should be captured from the node’s cached attributes (_content and _provider) without invoking the provider.

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[lydakis]

@codex

[chatgpt-codex-connector]

Summary

• Added a regression test that mounts a provider-backed file which raises when invoked, ensuring VirtualFileSystem.write_file can still overwrite the file without triggering the provider and that subsequent reads return the new content. tests/test_vfs.pyL42-L59

Testing

• ✅ uv run pytest

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