Merge remote-tracking branch 'origin/issue-915' into integration_2025_w12 (!3319)

Disable EPS NAS security algorithms in 5GMM UE capabilities

OAI nrUE is not supporting multiple RATs, therefore the EPS NAS security
algorithms bit in 5GMM UE capabilities is not relevant.

This bit is only relevant if N26 interface is supported by the AMF and
the UE is supporting S1 mode, meaning that the UE is connected to a 4G
LTE network,  via the S1 interface (eNB - MME/SGW). When the UE performs
an inter-system change from N1 mode to S1 mode, it transitions from 5GC
to the EPC: at network level this happens over the N26 interface (which
connects the 5G AMF to the 4G MME).

In this transition is where the EPS security context becomes relevant:
the UE does not know about the N26 interface, however is the AMF that
can tell the UE, e.g. during the registration procedure, if the UE sets
the S1 mode bit to "S1 mode supported" in the 5GMM capability IE, and
the AMF supports the N26 interface, the AMF will include the Selected
EPS NAS security algorithms IE in the SECURITY MODE COMMAND message,
which is what happened in #915.

Closes: #915

openair3/NAS/NR_UE/nr_nas_msg.c

@@ -520,7 +520,7 @@ static FGMMCapability set_fgmm_capability(nr_ue_nas_t *nas)
   cap.restrict_ec = 0;
   cap.lpp = 1;
   cap.ho_attach = 1;
-  cap.s1_mode = 1;
+  cap.s1_mode = 0;
 
   if (cap.length == 1)
     return cap; // Send minimum length only, 1 octet