Fix N2 handover integrity failures by enabling ciphering during handover setup

Problem:
During N2 handover with integrity protection (NIA1/NIA2), the target gNB
was disabling ciphering when configuring SRB1 security, while the UE
continued using the old security context from the source gNB until it
received and processed the RRC Reconfiguration with masterKeyUpdate. The
first PDUs sent after CFRA were still ciphered with the old keys, but the
target gNB had ciphering disabled, causing it to skip deciphering and
attempt integrity verification on ciphered data, leading to MAC-I
mismatches and integrity failures.

Specification Reference:
According to TS 33.501 section 6.11, "the UE shall keep the K gNB used in
the source cell until the handover or a connection re-establishment has been
completed successfully". This means the UE continues using the old security
context (old KRRCint, KRRCenc) until it receives and processes the RRC
Reconfiguration with masterKeyUpdate. The target gNB must therefore enable
ciphering during handover setup to correctly decipher and verify integrity
of PDUs sent with the old security context before the UE switches to the new
keys.

Root Cause:
The code was calling nr_rrc_pdcp_config_security(UE, false) during
handover setup, which disabled ciphering. This was incorrect because:
1. The UE already has security active from the source gNB
2. The UE continues using old ciphered keys until masterKeyUpdate
3. The UE sends ciphered PDUs immediately after CFRA succeeds
4. The target gNB must be able to decipher these PDUs to verify integrity

Why it only manifested when drb_integrity = "yes":
The bug was always present, but only caused visible failures when
integrity verification was enabled. When drb_integrity = "no", the config
typically also sets integrity_algorithms to only NIA0, resulting in
UE->integrity_algorithm = 0, which causes has_integrity = 0 for SRB1,
skipping integrity verification and hiding the underlying ciphering bug.
When drb_integrity = "yes", integrity_algorithms typically includes
NIA1/NIA2, resulting in has_integrity = 1 for SRB1, causing integrity
checks to run on ciphered data and fail.

Fix:
Change nr_rrc_pdcp_config_security(UE, false) to
nr_rrc_pdcp_config_security(UE, true) during handover setup to enable
ciphering immediately, allowing the target gNB to correctly decipher and
verify integrity of PDUs sent with the old security context.

Note: This fix is specific to handover. During initial context setup,
ciphering should remain disabled until SecurityModeComplete is received,
as the UE hasn't activated ciphering yet at that point.

openair2/RRC/NR/rrc_gNB_NGAP.c

@@ -1192,7 +1192,11 @@ int rrc_gNB_process_Handover_Request(gNB_RRC_INST *rrc, instance_t instance, nga
   // Activate SRBs
   activate_srb(UE, SRB1);
   activate_srb(UE, SRB2);
-  nr_rrc_pdcp_config_security(UE, false);
+  // During N2 handover, the UE continues using the old security context from the source gNB
+  // until it receives and processes the RRC Reconfiguration with masterKeyUpdate. The first
+  // PDUs sent after CFRA are still ciphered with the old keys. The target gNB must enable
+  // ciphering during handover setup to correctly decipher and verify integrity of these PDUs.
+  nr_rrc_pdcp_config_security(UE, true);
 
   // Process all PDU Session Resource Setup items from handover request
   DevAssert(msg->nb_of_pdusessions <= NGAP_MAX_PDU_SESSION);