Add readonly where it is missing and fix those field names according to their modifiers (#2589)

Content.Server/AI/Operators/Combat/Melee/SwingMeleeWeaponOperator.cs

@@ -9,7 +9,7 @@ namespace Content.Server.AI.Operators.Combat.Melee
 {
     public class SwingMeleeWeaponOperator : AiOperator
     {
-        private float _burstTime;
+        private readonly float _burstTime;
         private float _elapsedTime;
 
         private readonly IEntity _owner;

Content.Server/AI/Operators/Combat/Melee/UnarmedCombatOperator.cs

@@ -8,7 +8,7 @@ namespace Content.Server.AI.Operators.Combat.Melee
 {
     public sealed class UnarmedCombatOperator : AiOperator
     {
-        private float _burstTime;
+        private readonly float _burstTime;
         private float _elapsedTime;
 
         private readonly IEntity _owner;