Fix various small errors when running in the Swift 6 language mode (#4109)

* Fix various small errors when running in the Swift 6 language mode

* Make the `TargetConfiguration` run on the main actor.

* Fixed a comment

* Add a comment as to why we can't make the whole NSE a main actor.

* Fix the unit tests

* Fix `blankLinesAtStartOfScope` swiftformat error.

ElementX/Sources/Services/Media/Provider/MediaFileHandleProxy.swift

@@ -6,11 +6,12 @@
 //
 
 import Foundation
-import MatrixRustSDK
+
+@preconcurrency import MatrixRustSDK
 
 /// A wrapper around Rust's `MediaFileHandle` type that provides us with a
 /// media file that is stored unencrypted in a temporary location for previewing.
-class MediaFileHandleProxy {
+final class MediaFileHandleProxy: Sendable {
     /// The underlying handle for the file.
     private let handle: MediaFileHandleProtocol
     

ElementX/Sources/Services/Media/Provider/MediaLoader.swift

@@ -7,9 +7,10 @@
 
 import Combine
 import Foundation
-import MatrixRustSDK
 import UIKit
 
+@preconcurrency import MatrixRustSDK
+
 private final class MediaRequest {
     var continuations: [CheckedContinuation<Data, Error>] = []
 }

ElementX/Sources/Services/Media/Provider/MediaSourceProxy.swift

@@ -6,9 +6,9 @@
 //
 
 import Foundation
-import MatrixRustSDK
+@preconcurrency import MatrixRustSDK
 
-struct MediaSourceProxy: Hashable {
+struct MediaSourceProxy: Hashable, Sendable {
     /// The media source provided by Rust.
     let underlyingSource: MediaSource
     /// The media's mime type, used when loading the media's file.