[dcsctp] Immediately send outstanding data after handover

During handover, in-flight chunks are correctly marked as
is_to_be_retransmitted = true so they don't stall waiting
for ACKs on the old connection. However, SendBufferedPackets
was missing on resumption, causing the socket to idle until the
T3-RTX timer naturally expired (1000ms by default) to flush
the queue.

This adds the missing explicitly flush and verifies it with a
regression test against the stall.

Bug: webrtc:477364365
Change-Id: I47bd79e929c96d624e17be42db65ea920309fb09
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504100
Reviewed-by: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Priya Rajarathinam <rajarathinam@google.com>
Cr-Commit-Position: refs/heads/main@{#48656}