[Vertex AI] Fix CountTokensResponse decoding (#14447)

Author: andrewheard

FirebaseVertexAI/Sources/CountTokensRequest.swift

@@ -64,4 +64,19 @@ extension CountTokensRequest: Encodable {
 }
 
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
-extension CountTokensResponse: Decodable {}
+extension CountTokensResponse: Decodable {
+  enum CodingKeys: CodingKey {
+    case totalTokens
+    case totalBillableCharacters
+    case promptTokensDetails
+  }
+
+  public init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+    totalTokens = try container.decodeIfPresent(Int.self, forKey: .totalTokens) ?? 0
+    totalBillableCharacters =
+      try container.decodeIfPresent(Int.self, forKey: .totalBillableCharacters)
+    promptTokensDetails =
+      try container.decodeIfPresent([ModalityTokenCount].self, forKey: .promptTokensDetails) ?? []
+  }
+}

FirebaseVertexAI/Sources/GenerateContentResponse.swift

@@ -312,15 +312,15 @@ extension GenerateContentResponse.UsageMetadata: Decodable {
   public init(from decoder: any Decoder) throws {
     let container = try decoder.container(keyedBy: CodingKeys.self)
     promptTokenCount = try container.decodeIfPresent(Int.self, forKey: .promptTokenCount) ?? 0
-    candidatesTokenCount = try container
-      .decodeIfPresent(Int.self, forKey: .candidatesTokenCount) ?? 0
+    candidatesTokenCount =
+      try container.decodeIfPresent(Int.self, forKey: .candidatesTokenCount) ?? 0
     totalTokenCount = try container.decodeIfPresent(Int.self, forKey: .totalTokenCount) ?? 0
-    promptTokensDetails = try container
-      .decodeIfPresent([ModalityTokenCount].self, forKey: .promptTokensDetails) ??
-      [ModalityTokenCount]()
-    candidatesTokensDetails = try container
-      .decodeIfPresent([ModalityTokenCount].self, forKey: .candidatesTokensDetails) ??
-      [ModalityTokenCount]()
+    promptTokensDetails =
+      try container.decodeIfPresent([ModalityTokenCount].self, forKey: .promptTokensDetails) ?? []
+    candidatesTokensDetails = try container.decodeIfPresent(
+      [ModalityTokenCount].self,
+      forKey: .candidatesTokensDetails
+    ) ?? []
   }
 }