fix(sdk): preserve filenames in fs.readdir

[mmarufov]

Closes #379.

fs.readdir() splits valid filenames on newlines and |, truncating names or producing extra entries such as ...

Use NUL-delimited find -printf output in both modes, with the type before the name for withFileTypes. Preserve filename whitespace, treat relative paths literally, and update the mock's formatter.

Compatibility

• Listings now include dotfiles and use consistent name ordering. Directory symlinks are followed in both modes; child symlinks retain their type.
• Upgrade @vercel/sandbox-mock together with @vercel/sandbox; older mocks cannot handle the new command. A patch changeset covers both packages.
• A regular-file input now returns [] in plain mode instead of [path]. Node-compatible ENOTDIR handling remains outside this change.

Validation

• Build and typecheck passed for the SDK, mock, and CLI; all 878 existing and new local tests passed.
• Regression checks fail with the original implementation and pass with the fix. Local GNU find checks also covered symlinks, empty/missing paths, and option-like paths.
• The targeted live compatibility test passed on c9a985a, verifying exact names and file types in both modes, directly and through a directory symlink. NUL output survived the real API, and the sandbox was confirmed stopped. The full integration suite was not run.

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