File Server Fixed Use After Free In Filestreamer Callbacks

Fixed a use-after-free in the file_server HTTP filter when a request is cancelled (downstream RST, stream timeout, etc.) before the underlying AsyncFileManager::stat or read callback fires. FileStreamer previously captured [this] into the async-file callbacks, which left a dangling pointer if the FileStreamer was destroyed while a stat or read was still in flight or already queued on the owner dispatcher. The callbacks now use cancelWrapped (source/common/common/cancel_wrapper.h): the in-flight file operation is aborted via the file-side cancel, and any callback already dispatched to the worker's event loop short-circuits before dereferencing the freed FileStreamer.