[AI] Nullable support in structured output and more integration tests

FirebaseAI/Sources/Protocols/Public/StructuredOutput/FirebaseGenerable.swift

@@ -40,6 +40,17 @@ extension Optional: ConvertibleToModelOutput where Wrapped: ConvertibleToModelOu
   }
 }
 
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+extension Optional: ConvertibleFromModelOutput where Wrapped: ConvertibleFromModelOutput {
+  public init(_ content: ModelOutput) throws {
+    if case .null = content.kind {
+      self = nil
+      return
+    }
+    self = try Wrapped(content)
+  }
+}
+
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 extension Bool: FirebaseGenerable {
   public static var jsonSchema: JSONSchema {

FirebaseAI/Sources/Types/Public/StructuredOutput/ModelOutput.swift

@@ -120,6 +120,10 @@ public struct ModelOutput: Sendable, CustomDebugStringConvertible, FirebaseGener
       return nil
     }
 
+    if case .null = value.kind {
+      return nil
+    }
+
     return try Value(value)
   }
 }

FirebaseAI/Tests/TestApp/Packages/FirebaseAILogicExtended/Sources/FirebaseAILogicMacros/FirebaseGenerableMacro.swift

@@ -183,7 +183,10 @@ extension FirebaseGenerableMacro: MemberMacro {
 
       propertySchemas.append(propertySchemaString)
 
-      let propertyTypeString = info.type.trimmed.description
+      var propertyTypeString = info.type.trimmed.description
+      if let optionalType = info.type.as(OptionalTypeSyntax.self) {
+        propertyTypeString = optionalType.wrappedType.trimmed.description
+      }
       partiallyGeneratedProperties
         .append("var \(info.name): \(propertyTypeString).Partial?")
       partiallyGeneratedInits