WKWebView customUserAgent replaces system User-Agent with NetworkingExtension identifier on iOS 26.4

Reproduction Steps:a. Create a WKWebView instance and set a custom string to customUserAgent.b. Load any web page (e.g., https://example.com).c. Check the User-Agent field in the request headers via packet capture tools or Web Inspector. Expected Result:The custom User-Agent should be appended to the default system identifier (Mozilla/5.0 (iPhone; CPU iPhone OS 18_7 like Mac OS X) AppleWebKit/605.1.15...), instead of being completely overwritten. Actual Result:The User-Agent is fully replaced with: NetworkingExtension/8624.1.16.10.6 Network/5812.102.3 iOS/26.4, and all basic system identifiers are missing. Additional Information: Device: iPhone 16 Pro Max iOS Version: 26.4 (Build 20T5127) WKWebView setup: Directly set using the customUserAgent property Network Extension features are not used in the project, but the NetworkingExtension identifier still appears in the User-Agent