feat: add redundant_final_actor rule

[VDurocher]

Description

Adds a new redundant_final_actor rule that warns when a final modifier is applied to an actor declaration.

Per SE-0306, actors are reference types without support for inheritance, so they are implicitly final. Writing final actor is therefore redundant, similar to how internal is redundant as an access modifier.

Closes #6407

Examples

Triggers:

final actor DataStorage {}

final actor Counter {
    var count = 0
}

public final actor Scheduler {}

Does not trigger:

actor DataStorage {}

public actor Scheduler {}

final class Foo {}  // still valid for classes

Auto-correction

This rule supports auto-correction: it removes the redundant final modifier automatically.

Notes

• BuiltInRules.swift updated manually (run make register to verify the generated output matches)
• CHANGELOG entry to be added under ### Enhancements in ## Main

[SwiftLintBot]

	15 Warnings
⚠️	This PR may need tests.
⚠️	This PR introduced a violation in DuckDuckGo: /SharedPackages/VPN/Sources/VPN/Diagnostics/NetworkProtectionConnectionBandwidthAnalyzer.swift:26:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in DuckDuckGo: /SharedPackages/VPN/Sources/VPN/Diagnostics/KeyExpirationTester.swift:27:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in DuckDuckGo: /SharedPackages/BrowserServicesKit/Sources/BrowserServicesKit/Watchdog/Watchdog.swift:26:8: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in DuckDuckGo: /SharedPackages/BrowserServicesKit/Sources/Networking/Auth/OAuthClient.swift:151:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in DuckDuckGo: /iOS/DuckDuckGo/AdAttribution/AdAttributionPixelReporter.swift:25:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in DuckDuckGo: /iOS/SharedTestUtils/Mocks/Core/Statistics/MockPixelFiring.swift:42:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in Swift: /stdlib/public/Concurrency/MainActor.swift:20:21: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in Swift: /stdlib/public/Concurrency/MainActor.swift:42:21: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in Wire: /wire-ios/Wire-iOS/Sources/UserInterface/Conversation/MessagesIndividualUpdatesFactory.swift:23:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in Wire: /WireMessaging/Sources/WireMessagingData/WireDrive/WireDriveNodeUploadManager.swift:23:9: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in Wire: /WireMessaging/Sources/WireMessagingData/WireDrive/NodesAPI/NodesAPI.swift:28:9: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in Wire: /wire-ios-data-model/Source/MLS/SubconversationGroupIDRepository.swift:68:8: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in WordPress: /WordPress/Classes/Services/BlockEditorCache.swift:6:1: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)
⚠️	This PR introduced a violation in WordPress: /Tests/KeystoneTests/Tests/Utility/ApplicationPasswordsRepositoryTests.swift:599:9: Warning: Actors are implicitly final; the final modifier is redundant. (redundant_final_actor)

	19 Messages
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[SimplyDanny]

Thank you! Unfortunately, I have to tell you that there is another PR already that I just haven't found the time to review. I would like to move that one forward.