This PR replaces the Get*CallbackAtIndex pattern in the PluginManager
with returning a snapshot of callbacks that the caller can iterate over
using a range-based for loop. This is a continuation of #184452 which
added thread safety by using snapshots. However, that introduced a bunch
of unnecessary copies which are largely eliminated again by getting the
snapshot once when gather all the callbacks, rather than doing that on
each iteration when querying a plugin for a given index. It also
eliminates the possibility of the snapshot changing underneath you when
iterating over the plugins.
This change was largely mechanical and I used Claude to do the menial
work of updating the signatures and call sites.
81a537e708