[UII] Generated integration policies do not use namespace from agent policy

[jen-huang]

When Fleet generates a Fleet Server policy, fleet_server and system integration policies are generated automatically and attached. However, they hard-code the default namespace rather than leaving it empty to fallback to what the parent agent policy sets. Compare this to what happens when an integration is manually added to this policy:

This also occurs for the generated system integration policy that are added to normal agent policies when user leaves "Collect system logs and metrics` checked:

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

There is a similar issue reported here, which had an open community pr #175870

[jen-huang]

Thanks @juliaElastic, I missed that. Seems that community PR has been abandoned so whoever picks up this work should clean up that issue and PR.