Fix nil pointer panic when cancelling an intercepted request

[SAY-5]

CancelRequest calls ModifyRequest(reqID, nil, nil) to abort an intercepted request, but ModifyRequest dereferences modReq (*modReq = *modReq.WithContext(...)) before checking it, so cancelling any intercepted request panics with a nil pointer dereference.

ClearRequests already aborts by sending nil straight to the request channel. This adds the same handling to ModifyRequest: when modReq is nil, forward nil to the channel as the abort signal instead of dereferencing it. The waiting RequestModifier then returns ErrRequestAborted as intended.

Added a regression test that cancels a pending intercepted request and confirms it no longer panics.

Fixes #145