fix(ai): restore pre-AOS berserk energy vortexes and blade spirits (#2666)

#2132 let a player's energy vortex or blade spirits in pre-AOS Felucca target their own caster, gated by `FollowsAcquireRules`. #2232's AI overhaul moved the summon checks into `IsInvalidSummonTarget`, which excluded the caster in every era, and dropped the gate. Since then `FollowsAcquireRules` has had no readers and summons have never turned on their caster.

## Changes

- `IsInvalidSummonTarget` reads `FollowsAcquireRules` again. A summon that follows the acquire rules spares its caster and anyone the caster's harmful spells would spare. The vortex and blade spirits don't follow them in pre-AOS Felucca when a player summoned them.
- The animated-dead restrictions (no players, no other animated dead, no pets) apply to every animated summon regardless of that setting.
- `AcquireNewFocusMob` sets a berserk summon's caster aside and targets them only when nobody else is available. #2132 meant to prefer others, but its check only worked when the caster was scanned before the other candidates, and the vortex's Int-plus-Magery ranking usually puts a nearby caster first.
- The AOS summon-target check no longer dereferences a missing `SummonMaster`. A summon whose caster was deleted before a reload comes back without one.

Controlled summons are unaffected: since #2665 they target through `HandleControlled` and never reach this scan.

## Tests

`SummonAcquireRulesTests` has 6 cases, skipped without client map data. Pre-AOS Felucca: a vortex and blade spirits each target their caster when the caster is alone, and a vortex targets a bystander even when the caster ranks higher. Pre-AOS Trammel and AOS Felucca: the caster is still spared. An AOS scan next to a summon with no master doesn't throw. `UOContent.Tests`: 1148 passed, 2 skipped, 0 failed on Windows.

Projects/UOContent/Mobiles/BaseCreature.cs

@@ -920,6 +920,10 @@ namespace Server.Mobiles
         [CommandProperty(AccessLevel.GameMaster)]
         public bool IsPrisoner { get; set; }
 
+        /// <summary>
+        /// When false, a summon may target its own caster and anyone the caster's harmful spells would spare.
+        /// It still prefers any other target over the caster.
+        /// </summary>
         public virtual bool FollowsAcquireRules => true;
 
         public virtual Faction FactionAllegiance => null;