fix(mcp): propagate network errors from replaceContainerSnapshot

[HoneyBBQ]

Summary

• Network setup failure after snapshot replace (rollback/commit) was silently swallowed — the container would start but remain unreachable via gRPC.
• Follows up on fix(containerd): prevent silent network failures from leaving containers unreachable #202 which fixed the same pattern in other code paths but missed replaceContainerSnapshot.

Changes

File	Fix
internal/mcp/versioning.go	Return error from SetupNetwork instead of logging a warning; also error when IP is empty

Test plan

• Create a snapshot, rollback — verify container is reachable via gRPC after rollback
• Simulate CNI failure during rollback — verify error is returned, not silently swallowed