dcsctp: Correct Fast Recovery retransmission logic

This commit fixes a logic error in OutstandingData that prevented the
correct application of Fast Recovery retransmission rules when the
Cumulative TSN Ack point advanced.

Previously, HandleSack would call RemoveAcked before
NackBetweenAckBlocks. Because RemoveAcked updates the internal
last_cumulative_tsn_ack_ to match the incoming SACK, the subsequent
check (cumulative_tsn_ack > last_cumulative_tsn_ack_) in
NackBetweenAckBlocks would always be false. This resulted in miss
indications not being incremented for all missing TSNs as required
during Fast Recovery.

Per RFC 9260 Section 7.2.4: "If an endpoint is in Fast Recovery and
a SACK chunks arrives that advances the Cumulative TSN Ack Point, the
miss indications are incremented for all TSNs reported missing in the
SACK chunk."

The fix involves determining if the cumulative ACK point is advancing
before the internal state is modified in HandleSack. A new unit test
FastRecoveryIncrementsNackCountWhenCumulativeTsnAdvances verifies
that missing TSNs correctly receive miss indications in this scenario.

No-Try: True
Bug: None
Change-Id: Ic8ef7026bfb3617bd2094bae47de13b9d0133930
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/448080
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46864}