fix(mount): handle snapshot root path

[ousamabenyounes]

What

Fix plakar mount -to <path> <snapshotID> when the selected snapshot path resolves to the snapshot root. The mount path preparation now keeps the full snapshot filesystem for / instead of calling fs.Sub with an empty path.

Closes #2086

Why

A root snapshot selection can produce / from locate.OpenSnapshotByPath; trimming that to an empty subpath makes fs.Sub return sub : invalid argument before the mount can proceed.

RED -> GREEN

RED, before the helper/fix existed:

# github.com/PlakarKorp/plakar/subcommands/mount [github.com/PlakarKorp/plakar/subcommands/mount.test]
subcommands/mount/mount_parse_test.go:56:16: undefined: snapshotSubFS
FAIL github.com/PlakarKorp/plakar/subcommands/mount [build failed]

GREEN, after implementing snapshotSubFS with root-path handling:

ok github.com/PlakarKorp/plakar/subcommands/mount 0.015s

Revert proof, after temporarily removing the / branch from snapshotSubFS:

--- FAIL: TestSnapshotSubFSRootPathUsesSnapshotRoot (0.00s)
    mount_parse_test.go:57:
        Error: Received unexpected error:
               sub : invalid argument
FAIL
FAIL github.com/PlakarKorp/plakar/subcommands/mount 0.016s

GREEN again after restoring the fix:

ok github.com/PlakarKorp/plakar/subcommands/mount 0.438s

Coverage added

• Added a root-path unit test for the mount snapshot filesystem helper.
• The test fails when the root-path branch is reverted, proving it catches [Bug]: mount: unable to mount a snapshot to a specific path #2086.

Test plan

• docker run --rm -v "$PWD":/src -v /tmp/plakar-go-cache:/go/pkg/mod -v /tmp/plakar-go-build:/root/.cache/go-build -w /src golang:1.25 go test ./subcommands/mount -run TestSnapshotSubFSRootPathUsesSnapshotRoot -count=1
• docker run --rm -v "$PWD":/src -v /tmp/plakar-go-cache:/go/pkg/mod -v /tmp/plakar-go-build:/root/.cache/go-build -w /src golang:1.25 go test ./subcommands/mount -count=1
• docker run --rm -v "$PWD":/src -v /tmp/plakar-go-cache:/go/pkg/mod -v /tmp/plakar-go-build:/root/.cache/go-build -w /src golang:1.25 sh -c 'go build -v ./... && go test ./...'

Full validation result:

exit:0
go build -v ./...: pass
go test ./...: pass