fix(handler): forward onBodySent and onRequestSent in DecoratorHandler and CacheHandler

[marko1olo]

Summary

DecoratorHandler.onBodySent was declared as a no-op () => {} instead of delegating to the wrapped handler. onRequestSent was not declared at all, so Request silently skipped calling it.

Since every built-in interceptor either extends DecoratorHandler (RedirectHandler, RetryHandler, …) or hand-forwards handler methods (CacheHandler), composing any interceptor onto a dispatcher silently dropped these two hooks for the user's handler.

Fixes #5695

Changes

• DecoratorHandler.onBodySent now forwards via optional chaining to this.#handler.onBodySent?.(...args)
• DecoratorHandler.onRequestSent added with the same forwarding pattern
• CacheHandler gains onBodySent and onRequestSent forwarding (was missing entirely)
• 4 new unit tests in test/decorator-handler.js covering delegation and graceful no-op when the inner handler omits the method

[marko1olo]

Closing — #5696 was merged in the meantime and covers the same fix. Thanks for the quick turnaround upstream!