fix: isolate REST timeout middleware handler context (#49982)

Related to #49981

The REST timeout middleware could return a timeout response while the
handler goroutine continued using the original Gin context. Once Gin
recycled that context, late handler writes could race with a later
request and trigger concurrent map writes.

Run timeout-wrapped handlers with a copied Gin context and a fully
buffered response recorder instead. The original context and real
response writer now remain owned by the middleware goroutine, while
normal completion explicitly propagates selected metadata and commits
the recorder. Timeout closes the recorder and writes the 408 response
through the real writer, so late handler writes are discarded safely.

Add tests for recorder isolation, normal buffered commits, metadata and
abort propagation, late writes after timeout, and race validation of the
late-write timeout path.

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