…r CPU waste

Replace IEnumerable<T> fields with List<T> to avoid deeply nested
ConcatIterator chains from repeated .Concat() calls. Each WithAdditionalTrigger
call now uses List.Add() instead. Also cache the service provider hash code,
use O(1) List.Count property instead of LINQ .Count(), and add count-based
fast paths in ShouldUseSameServiceProvider to short-circuit before SequenceEqual.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit 0cc6aa1)