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.

Related to #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.

Original issue repro status

I re-ran the exact issue steps on unmodified origin/main in Docker/Linux. The original symptom was not reproduced in this environment: the command got past the fs.Sub path and failed later at FUSE mount setup.

snapshot=7adcc28f
/tmp/plakar: mount: fusermount: exit status 1
2026/07/16 16:05:09 mount helper error: fusermount: mount failed: Permission denied

Full repro log: /tmp/see-real-bug_PlakarKorp-plakar_2086_rerun.log.

So this PR should be treated as a targeted root-path regression fix, not as a fully confirmed reproduction of #2086 on Linux. The issue reporter reproduced on macOS; I do not have a macOS/FUSE environment here to confirm the original plakar: sub : invalid argument symptom end-to-end.

Regression proof (RED -> GREEN)

The regression test exercises the failing root-path branch directly.

RED, 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 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 covers the root-path regression.

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