[Vertex AI] Fix decoding ModalityTokenCount when tokenCount is 0

Author: andrewheard

FirebaseVertexAI/Sources/ModalityTokenCount.swift

@@ -57,5 +57,18 @@ public struct ContentModality: DecodableProtoEnum, Hashable, Sendable {
     VertexLog.MessageCode.generateContentResponseUnrecognizedContentModality
 }
 
+// MARK: Codable Conformances
+
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
-extension ModalityTokenCount: Decodable {}
+extension ModalityTokenCount: Decodable {
+  enum CodingKeys: CodingKey {
+    case modality
+    case tokenCount
+  }
+
+  public init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+    self.modality = try container.decode(ContentModality.self, forKey: .modality)
+    self.tokenCount = try container.decodeIfPresent(Int.self, forKey: .tokenCount) ?? 0
+  }
+}