Add x-ios-bundle-identifier header to GenerativeAIService requests

This change adds the `x-ios-bundle-identifier` header to requests made by `GenerativeAIService`. The value is set to `Bundle.main.bundleIdentifier` if available. This header is sent unconditionally.

A test assertion has been added to `GenerativeModelTestUtil.swift` to verify the presence and correctness of this header in unit tests.

Author: google-labs-jules[bot]

FirebaseAI/Sources/GenerativeAIService.swift

@@ -211,6 +211,10 @@ struct GenerativeAIService {
       }
     }
 
+    if let bundleID = Bundle.main.bundleIdentifier {
+      urlRequest.setValue(bundleID, forHTTPHeaderField: "x-ios-bundle-identifier")
+    }
+
     let encoder = JSONEncoder()
     urlRequest.httpBody = try encoder.encode(request)
     urlRequest.timeoutInterval = request.options.timeout

FirebaseAI/Tests/Unit/TestUtilities/GenerativeModelTestUtil.swift

@@ -69,6 +69,9 @@ enum GenerativeModelTestUtil {
         XCTAssertEqual(firebaseAppID, dataCollection ? "My app ID" : nil)
         XCTAssertEqual(appVersion, dataCollection ? expectedAppVersion : nil)
 
+        let bundleID = request.value(forHTTPHeaderField: "x-ios-bundle-identifier")
+        XCTAssertEqual(bundleID, Bundle.main.bundleIdentifier)
+
         if let authToken {
           XCTAssertEqual(
             request.value(forHTTPHeaderField: "Authorization"),