Fix crash when response completes before request body finishes uploading

[fabianfett]

• Fix a fatalError("Invalid state: idle") crash in HTTP1ConnectionStateMachine.demandMoreResponseBodyParts() that occurs when a response completes before the request body finishes uploading
• The root cause is that self.request was only nilled out inside the write-completion callback for .sendRequestEnd, creating a window where demandResponseBodyStream could still see the old request and call into the state machine after it had already transitioned to .idle
• The fix nils out self.request synchronously when handling .sendRequestEnd (before the write completes), and moves requestBodyStreamSent() to fire after the final action rather than before it