Add resilient AI service handling

Tests/MusicAssistantCoreTests/OpenAIClientTests.swift

@@ -128,7 +128,7 @@ final class OpenAIClientTests: XCTestCase {
         XCTAssertEqual(recordedRequestCount, 0)
     }
 
-    func testClientReportsUnacceptableStatusWithoutLeakingResponseBody() async throws {
+    func testClientReportsRateLimitWithoutLeakingResponseBody() async throws {
         let transport = RecordingOpenAITransport(
             statusCode: 429,
             responseData: Data("secret server detail".utf8)
@@ -150,7 +150,32 @@ final class OpenAIClientTests: XCTestCase {
             )
             XCTFail("Expected status error to throw.")
         } catch let error as OpenAIClientError {
-            XCTAssertEqual(error, .unacceptableStatusCode(429))
+            XCTAssertEqual(error, .rateLimited(retryAfter: nil))
+        }
+    }
+
+    func testClientReadsRetryAfterForRateLimitedResponses() async throws {
+        let transport = RecordingOpenAITransport(
+            statusCode: 429,
+            responseData: Data(),
+            responseHeaders: ["Retry-After": "3.5"]
+        )
+        let client = OpenAIClient(
+            configuration: OpenAIClientConfiguration(
+                apiKey: "test-api-key",
+                endpointURL: URL(string: "https://api.example.test/v1/configured")!
+            ),
+            adapter: MockOpenAIClientAdapter(),
+            transport: transport
+        )
+
+        do {
+            _ = try await client.generateSongProject(
+                from: SongProjectGenerationRequest(context: AIRequestContext(userInstruction: "Generate."))
+            )
+            XCTFail("Expected rate limit error to throw.")
+        } catch let error as OpenAIClientError {
+            XCTAssertEqual(error, .rateLimited(retryAfter: 3.5))
         }
     }
 }
@@ -159,10 +184,16 @@ private actor RecordingOpenAITransport: OpenAIHTTPTransport {
     private(set) var recordedRequests: [URLRequest] = []
     private let statusCode: Int
     private let responseData: Data
+    private let responseHeaders: [String: String]?
 
-    init(statusCode: Int, responseData: Data) {
+    init(
+        statusCode: Int,
+        responseData: Data,
+        responseHeaders: [String: String]? = nil
+    ) {
         self.statusCode = statusCode
         self.responseData = responseData
+        self.responseHeaders = responseHeaders
     }
 
     var requestBodyString: String? {
@@ -176,7 +207,7 @@ private actor RecordingOpenAITransport: OpenAIHTTPTransport {
             url: request.url!,
             statusCode: statusCode,
             httpVersion: "HTTP/1.1",
-            headerFields: nil
+            headerFields: responseHeaders
         )!
         return (responseData, response)
     }