fix copyFilesystems raw-copy branch; cover with tests

[eriknordmark]

Summary

Two related changes:

Bug fix: raw-copy branch was unreachable

copyFilesystems detects "no recognized filesystem on source" via:

case err != nil && !errors.Is(err, &disk.UnknownFilesystemError{}):

But go-diskfs returns a fresh *UnknownFilesystemError with a populated partition field (disk.NewUnknownFilesystemError(partIndex)). errors.Is compares against the zero-valued sentinel &UnknownFilesystemError{} and never matches, so any source partition without a recognized filesystem (including a real squashfs we'd want to grow) falls into the rejection arm above instead of the raw-copy arm below.

Switched to an errors.As-based helper (isUnknownFilesystem) that ignores the per-instance field. This makes the raw-copy / squashfs path actually reachable, which is the path needed for partitions whose source is squashfs (a common shape: rootfs images grown to make room for larger replacements).

Test coverage for previously-uncovered paths

Four additions:

• TestSwapPartitions — confirms swapPartitions round-trips Name / Type-GUID / PartUUID / Attributes between the original slot and the alt-labeled new slot, and preserves geometry. Catches inversion-on-double-call regressions.
• TestShrinkFilesystems — implements the previously-empty stub. Mocks execResize2fs and covers: skip when current ≤ target, invocation when shrinking, error propagation, and rejection of non-ext4 sources.
• TestCopyFilesystemsRawCopy — exercises the freshly-reachable raw-copy branch via an unrecognized-filesystem source. Uses equal-sized partitions to avoid an unrelated go-diskfs verification bug (sync.verifyBlockCopy rejects target-larger-than-source; filed at sync.verifyBlockCopy fails when target partition is larger than source go-diskfs#403).
• TestCreatePartitions update — corrects the expectation: createPartitions intentionally assigns the alternate label (<orig>_resized2) to the new slot; a later swapPartitions step is what restores the original name. The previous expectation asserted the post-swap state but was running before swap. Verified passes against both unmodified upstream go-diskfs and a local patched build.

Test results

ok  	github.com/diskfs/partitionresizer  ...
ok  	github.com/diskfs/partitionresizer/cmd/resizer

(Pre-existing TestRun failure remains; that's blocked on an unrelated go-diskfs ext4 issue — diskfs/go-diskfs#402 — and not in scope here.)

Notes for review

• Per-phase functions (shrinkFilesystems, createPartitions, copyFilesystems, swapPartitions, removePartitions) remain unexported. Callers that want explicit per-phase orchestration would need them exported; that's a separate API question, not part of this PR. Discussed briefly in Add idempotency #2 (comment).

Signed-off-by: eriknordmark erik@zededa.com

[deitch]

Looks like linting errors.