Add redundant_final_actor rule

Actors in Swift cannot be subclassed (SE-0306), making the `final`
modifier redundant on actor declarations. This new opt-in rule detects
and auto-corrects `final actor` to just `actor`.

Examples:
  - `final actor MyActor {}` → `actor MyActor {}`
  - `public final actor DataStore {}` → `public actor DataStore {}`

Closes #6407

Author: william-laverty

Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift

@@ -176,6 +176,7 @@ public let builtInRules: [any Rule.Type] = [
     ReduceBooleanRule.self,
     ReduceIntoRule.self,
     RedundantDiscardableLetRule.self,
+    RedundantFinalActorRule.self,
     RedundantNilCoalescingRule.self,
     RedundantObjcAttributeRule.self,
     RedundantSelfRule.self,

Source/SwiftLintBuiltInRules/Rules/Idiomatic/RedundantFinalActorRule.swift

@@ -0,0 +1,63 @@
+import SwiftSyntax
+
+@SwiftSyntaxRule(explicitRewriter: true)
+struct RedundantFinalActorRule: OptInRule {
+    var configuration = SeverityConfiguration<Self>(.warning)
+
+    static let description = RuleDescription(
+        identifier: "redundant_final_actor",
+        name: "Redundant Final on Actor",
+        description: "`final` is redundant on an actor declaration because actors cannot be subclassed",
+        kind: .idiomatic,
+        nonTriggeringExamples: [
+            Example("actor MyActor {}"),
+            Example("final class MyClass {}"),
+            Example("""
+            @globalActor
+            actor MyGlobalActor {}
+            """),
+        ],
+        triggeringExamples: [
+            Example("↓final actor MyActor {}"),
+            Example("public ↓final actor DataStore {}"),
+            Example("""
+            @globalActor
+            ↓final actor MyGlobalActor {}
+            """),
+        ],
+        corrections: [
+            Example("final actor MyActor {}"):
+                Example("actor MyActor {}"),
+            Example("public final actor DataStore {}"):
+                Example("public actor DataStore {}"),
+        ]
+    )
+}
+
+private extension RedundantFinalActorRule {
+    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
+        override func visitPost(_ node: ActorDeclSyntax) {
+            if let finalModifier = node.modifiers.first(where: { $0.name.text == "final" }) {
+                violations.append(finalModifier.positionAfterSkippingLeadingTrivia)
+            }
+        }
+    }
+
+    final class Rewriter: ViolationsSyntaxRewriter<ConfigurationType> {
+        override func visit(_ node: ActorDeclSyntax) -> DeclSyntax {
+            guard let finalIndex = node.modifiers.firstIndex(where: { $0.name.text == "final" }) else {
+                return super.visit(node)
+            }
+            numberOfCorrections += 1
+            var modifiers = node.modifiers
+            modifiers.remove(at: finalIndex)
+            // If no modifiers remain, preserve the leading trivia on the actor keyword
+            var result = node.with(\.modifiers, modifiers)
+            if modifiers.isEmpty {
+                let leadingTrivia = node.modifiers[finalIndex].leadingTrivia
+                result = result.with(\.actorKeyword.leadingTrivia, leadingTrivia)
+            }
+            return super.visit(result)
+        }
+    }
+}