Merge remote-tracking branch 'origin/e1ap-issue-878' into integration_2025_w04 (!3171)

- Refactor SCTP Association Response Handling
  - Unified function for both CU-CP and CU-UP handling
  - CU-CP: Notifies RRC of E1 connection loss with CU-UP
  - CU-UP: Initiates a new SCTP association request to regain E1 connection
  - this fix will prevent issue #878 from happening

- Send list of PDU Session Resources to Failed Setup in the (PDU Session
  Resource/Initial Context) Setup Response
  - depending on different failure scenarios
  - e.g. (#878) when No CU-UP is associated and still the CU-CP receives a PDU
    Session Setup Request
  - add relevant functions to handle NGAP Setup Response messages with failed
    list of items
  - this is an extra failure handling to make #878 is handled if it ever happens
    again

- NGAP Cause Refactoring
  - Refactor struct for NGAP cause group and value
  - Add extended radio network cause enumeration values
  - Refactor redundant code into a new NGAP cause encoding function

- Minor logging, function descriptions improvements

Closes #878

doc/E1AP/E1-design.md

@@ -194,3 +194,7 @@ Note that CU-UPs are not released from CU-CP internal structures. That means
 that you have to restart the CU-CP if you want to connect the CU-UP again
 (e.g., after a crash). The CU-CP might also misfunction during attach if a
 CU-UP was connected, but disconnected in the meantime.
+
+# 5. Abnormal conditions
+
+* The CU-UP goes offline during normal operation (e.g. UEs have a valid PDU Session and are exchanging data on the UP): after restarting the CU-UP, the UP is not restored and the user will notice GTP errors. In this case the UEs have to reconnect.