Fixing Issues with Async Upstream (#108)

### Motivation

Fixing Issues with Building in Swift 6 for Linux

### Modifications

* Fix Issues with Upstream Iterator

### Result

Compatability with Swift 6 on Linux

### Test Plan

TBD

Sources/OpenAPIRuntime/Multipart/MultipartFramesToRawPartsSequence.swift

@@ -342,8 +342,17 @@ extension MultipartFramesToRawPartsSequence {
             switch stateMachine.nextFromPartSequence() {
             case .returnNil: return nil
             case .fetchFrame:
+                let frame: Upstream.AsyncIterator.Element?
                 var upstream = upstream
-                let frame = try await upstream.next()
+                #if compiler(>=6.0)
+                if #available(macOS 15, iOS 18.0, tvOS 18.0, watchOS 11.0, macCatalyst 18.0, visionOS 2.0, *) {
+                    frame = try await upstream.next(isolation: self)
+                } else {
+                    frame = try await upstream.next()
+                }
+                #else
+                frame = try await upstream.next()
+                #endif
                 self.upstream = upstream
                 switch stateMachine.partReceivedFrame(frame) {
                 case .returnNil: return nil
@@ -365,8 +374,17 @@ extension MultipartFramesToRawPartsSequence {
             switch stateMachine.nextFromBodySubsequence() {
             case .returnNil: return nil
             case .fetchFrame:
+                let frame: Upstream.AsyncIterator.Element?
                 var upstream = upstream
-                let frame = try await upstream.next()
+                #if compiler(>=6.0)
+                if #available(macOS 15, iOS 18.0, tvOS 18.0, watchOS 11.0, macCatalyst 18.0, visionOS 2.0, *) {
+                    frame = try await upstream.next(isolation: self)
+                } else {
+                    frame = try await upstream.next()
+                }
+                #else
+                frame = try await upstream.next()
+                #endif
                 self.upstream = upstream
                 switch stateMachine.bodyReceivedFrame(frame) {
                 case .returnNil: return nil