Fix parsing async keywords in void methods (#290)

## Overview

Methods returning `Void` implicitly aren’t parsed correctly for
declaration keywords like `throws` and `async` which can lead to
malformed code if the method body contains “async.”

```swift
func notAsync() {
  DispatchQueue.main.async {}
}
```

## Test Plan

Added an additional test case to check for async false positives.

Sources/MockingbirdGenerator/Parser/Models/Method.swift

@@ -154,6 +154,7 @@ struct Method {
     // Parse return type attributes.
     let returnAttributesStartIndex = parametersEndIndex ?? declaration.startIndex
     let returnAttributesEndIndex = declaration.firstIndex(of: "-", excluding: .allGroups)
+      ?? declaration.firstIndex(of: "{", excluding: .allGroups) // Void methods
       ?? declaration.endIndex
     let returnAttributes = declaration[returnAttributesStartIndex..<returnAttributesEndIndex]
     if returnAttributes.range(of: #"\basync\b"#, options: .regularExpression) != nil {

Sources/MockingbirdTestsHost/AsyncMethods.swift

@@ -20,4 +20,31 @@ protocol AsyncProtocol {
   @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
   func asyncClosureThrowingMethod(block: () async throws -> Bool) async throws -> Bool
 }
+
+class AsyncClass {
+  @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+  func asyncMethodVoid() async { fatalError() }
+
+  @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+  func asyncMethod() async -> Bool  { fatalError() }
+
+  @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+  func asyncMethod(parameter: String) async -> Int  { fatalError() }
+
+  @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+  func asyncThrowingMethod() async throws -> Int  { fatalError() }
+
+  @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+  func asyncClosureMethod(block: () async -> Bool) async  { fatalError() }
+
+  @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+  func asyncClosureThrowingMethod(block: () async throws -> Bool) async throws -> Bool {
+    fatalError()
+  }
+
+  // Test for false positives.
+  func notAsync() {
+    DispatchQueue.main.async {}
+  }
+}
 #endif