fix(atif): link system subagents to emitted parent spans (#15585)

* fix(atif): link system subagents to emitted parent spans

* fix(atif): ensure subagent parents are emitted

* test(atif): assert subagent root fallback hierarchy

* test(atif): cover validated batch conversion

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Co-authored-by: Elizabeth Hutton <42793432+ehutt@users.noreply.github.com>