Fix root predicates - #98942

Fix the definition of GC roots to account for all transient references

There was a categorical mistake in how uppers and collectible_dependents were interpreted. Both of these refer to a task above us and this is it is a reference but the reference might be comming from transient or persistent tasks. This is what kept confusing the 'root' definition

So the new definition is simply

• You are collectible if there are no references (persistent or transient)
• You are a root if there are only transient references, which includes transient uppers and collectible_dependents

The old definition expected roots to have no uppers at all, which isn't correct. A run_once task becomes an aggregation root and so the nested persistent tasks reference it as an upper. Those tasks should become gc roots since they are on the transient <--> persistent boundary and the only thing retaining them is references from transient tasks.