Fix launchctl bootstrap failures being discarded as XPC errors

[rogerneal]

Summary

• Surface launchctl bootstrap exit status and stderr from ServiceManager.register instead of continuing into a misleading apiserver XPC error.
• When running as root (euid == 0) outside an Aqua session, bootstrap into the system domain instead of user/0 or gui/0.
• Keep container system start idempotent when the service is already registered.
• Add unit tests for domain selection and a real launchctl failure path for error surfacing.

Fixes #2008

Test plan

Tested

• swift test --filter ServiceManagerTests (domain selection cases)
• Real launchctl failure via ServiceManager.register with a missing plist; error includes launchctl bootstrap and status
• Local Aqua bin/container system start --app-root ...: register() loaded com.apple.container.apiserver into gui/<uid> and the process started

Not tested

• Exact GitHub Actions macos-26 reproduction: sudo container system start in a Background / non-login session
• End-to-end healthy XPC ping / container system status after start in that CI environment
• Passwordless/sudo launchctl bootstrap system on this developer machine (not available in the local session used for this PR)

Suggested CI verification

On a macos-26 runner:

1. Install or build this branch
2. Run sudo container system start --enable-kernel-install
3. Confirm start succeeds without XPC connection error: Connection invalid
4. Run container system status (or another command that talks to the apiserver)