image scp: preserve usernames containing an "@"

[ROKUMATE]

Problem

podman image scp parses the user@localhost::image argument by cutting on the
first @. When the username itself contains an @ — e.g. an Active Directory
account like user@domain — it gets truncated to just user:

$ sudo podman image scp aduser@domain@localhost::example.com/foo/bar:latest
Error: user: unknown user aduser

Impact

Users on AD-joined (or any user@domain) machines can't use podman image scp
to copy images between users — the local user lookup fails before the transfer
even starts.

Fix

In ParseImageSCPArg, the @localhost:: case only ever matches that exact
separator, so split the argument on @localhost:: instead of the bare @. This
keeps the full username (including any @domain) and leaves the remote
user@host:: SSH path unchanged.

user@localhost::img          -> user        (unchanged)
user@domain@localhost::img   -> user@domain  (fixed)

Test

Added TestParseImageSCPArg in pkg/domain/utils/scp_test.go covering both the
plain user and the user@domain cases. go test ./pkg/domain/utils/ passes.

Fixes: #27655

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[danishprakash]

Overall LGTM, but can you squash your commits into one and sign it?

[ROKUMATE]

check now

[Honny1]

I did a fast check, and it seems good. Can you please add an integration test?

[ROKUMATE]

Added the integration test as well 👍

[runiq]

OP from the original issue here, I'd just like to say thanks a lot for implementing this ❤️

[ROKUMATE]

@runiq thanks to you to for helping me find my first issue for contributation 🙇🏼