fix: preserve REST request cancellation in size middleware (#49895)

Related to #49894

restfulSizeMiddleware wrapped metrics stats around the Gin context,
which can drop the original HTTP request cancellation signal before
timeoutMiddleware for v2 entity RESTful APIs.

Use the original request context as the metrics parent so client
disconnects continue propagating through the timeout and proxy task
chain. Add regression coverage for the
restfulSizeMiddleware(timeoutMiddleware(...)) chain.

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>