feat(chat): add completion_tokens_details to CompletionUsage

OpenAI's chat completion `usage` payload includes a
`completion_tokens_details` object for reasoning models, audio
outputs, and Predicted Outputs. The library was silently dropping
this on decode, making fields like reasoning_tokens unreachable.

Mirror the existing `PromptTokensDetails` shape with a new
`CompletionTokensDetails` struct exposing accepted_prediction_tokens,
audio_tokens, reasoning_tokens, and rejected_prediction_tokens. Fields
are `Int?` because each only appears under specific conditions
(reasoning_tokens for o-series, audio_tokens for audio output, the
prediction fields for Predicted Outputs).

The new init parameter has a default value, so existing call sites in
tests continue to compile unchanged.

Closes #258

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: Krivoblotsky

Sources/OpenAI/Public/Models/ChatResult.swift

@@ -269,24 +269,72 @@ public struct ChatResult: Codable, Equatable, Sendable {
         public let totalTokens: Int
         /// Breakdown of tokens used in the prompt.
         public let promptTokensDetails: PromptTokensDetails?
-        
+        /// Breakdown of tokens used in the completion.
+        public let completionTokensDetails: CompletionTokensDetails?
+
+        public init(
+            completionTokens: Int,
+            promptTokens: Int,
+            totalTokens: Int,
+            promptTokensDetails: PromptTokensDetails? = nil,
+            completionTokensDetails: CompletionTokensDetails? = nil
+        ) {
+            self.completionTokens = completionTokens
+            self.promptTokens = promptTokens
+            self.totalTokens = totalTokens
+            self.promptTokensDetails = promptTokensDetails
+            self.completionTokensDetails = completionTokensDetails
+        }
+
         public struct PromptTokensDetails: Codable, Equatable, Sendable {
             /// Audio input tokens present in the prompt.
             public let audioTokens: Int
             /// Cached tokens present in the prompt.
             public let cachedTokens: Int
-            
+
             enum CodingKeys: String, CodingKey {
                 case audioTokens = "audio_tokens"
                 case cachedTokens = "cached_tokens"
             }
         }
-        
+
+        public struct CompletionTokensDetails: Codable, Equatable, Sendable {
+            /// When using Predicted Outputs, the number of tokens in the prediction that appeared in the completion.
+            public let acceptedPredictionTokens: Int?
+            /// Audio output tokens generated by the model.
+            public let audioTokens: Int?
+            /// Tokens generated by the model for reasoning. Only present for reasoning models.
+            public let reasoningTokens: Int?
+            /// When using Predicted Outputs, the number of tokens in the prediction that did not appear in the completion.
+            /// However, like reasoning tokens, these tokens are still counted in the total completion tokens for purposes of billing, output, and context window limits.
+            public let rejectedPredictionTokens: Int?
+
+            public init(
+                acceptedPredictionTokens: Int? = nil,
+                audioTokens: Int? = nil,
+                reasoningTokens: Int? = nil,
+                rejectedPredictionTokens: Int? = nil
+            ) {
+                self.acceptedPredictionTokens = acceptedPredictionTokens
+                self.audioTokens = audioTokens
+                self.reasoningTokens = reasoningTokens
+                self.rejectedPredictionTokens = rejectedPredictionTokens
+            }
+
+            enum CodingKeys: String, CodingKey {
+                case acceptedPredictionTokens = "accepted_prediction_tokens"
+                case audioTokens = "audio_tokens"
+                case reasoningTokens = "reasoning_tokens"
+                case rejectedPredictionTokens = "rejected_prediction_tokens"
+            }
+        }
+
         enum CodingKeys: String, CodingKey {
             case completionTokens = "completion_tokens"
             case promptTokens = "prompt_tokens"
             case totalTokens = "total_tokens"
             case promptTokensDetails = "prompt_tokens_details"
+            case completionTokensDetails = "completion_tokens_details"
         }
     }
 }

Tests/OpenAITests/OpenAITestsDecoder.swift

@@ -75,7 +75,54 @@ class OpenAITestsDecoder: XCTestCase {
         )
         try decode(data, expectedValue)
     }
-    
+
+    func testCompletionUsageWithCompletionTokensDetails() async throws {
+        let data = """
+        {
+          "completion_tokens": 320,
+          "prompt_tokens": 80,
+          "total_tokens": 400,
+          "completion_tokens_details": {
+            "accepted_prediction_tokens": 12,
+            "audio_tokens": 0,
+            "reasoning_tokens": 256,
+            "rejected_prediction_tokens": 4
+          }
+        }
+        """
+
+        let expectedValue = ChatResult.CompletionUsage(
+            completionTokens: 320,
+            promptTokens: 80,
+            totalTokens: 400,
+            promptTokensDetails: nil,
+            completionTokensDetails: .init(
+                acceptedPredictionTokens: 12,
+                audioTokens: 0,
+                reasoningTokens: 256,
+                rejectedPredictionTokens: 4
+            )
+        )
+        try decode(data, expectedValue)
+    }
+
+    func testCompletionUsageDecodesWithoutCompletionTokensDetails() async throws {
+        let data = """
+        {
+          "completion_tokens": 12,
+          "prompt_tokens": 9,
+          "total_tokens": 21
+        }
+        """
+
+        let expectedValue = ChatResult.CompletionUsage(
+            completionTokens: 12,
+            promptTokens: 9,
+            totalTokens: 21
+        )
+        try decode(data, expectedValue)
+    }
+
     func testImageQuery() async throws {
         let imageQuery = ImagesQuery(
             prompt: "test",