Treat missing device data dir as success when forgetting a device

[mvanhorn]

Summary

parsec-cli device forget-local no longer errors with Failed to remove device data: Failed to remove data: No such file or directory (os error 2) when the device has no data directory. remove_device_data now treats ErrorKind::NotFound from remove_dir_all as success, making the cleanup idempotent; any other io error still propagates as before.

Why this matters

#12807 reports the confusing hard error on what is actually a successful forget operation - the device key file is removed correctly, but the follow-up data-dir removal fails. AureliaDolo noted in-thread this affects devices created on old organizations, which never got a per-device data directory. The fix is localized to the storage cleanup helper (libparsec/crates/platform_storage/src/native/cleanup.rs); the CLI command and client wrapper are unchanged.

How to test: parsec-cli device forget-local on a device whose <data_base_dir>/<device_id_hex>/ directory does not exist completes without the error; with the directory present, it is removed exactly as before.

Testing

• New cleanup_device_data_missing_dir test: forgetting a device whose data dir was never created returns Ok (this fails on master with NotFound)
• Existing cleanup_device_data still passes (directory present and removed)
• cargo test -p libparsec_platform_storage --lib --features test-with-testbed cleanup: 3 passed
• News fragment added: newsfragments/12807.bugfix.rst

Fixes #12807

[FirelightFlagboy]

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