[nexus] improve robustness of MLE and WPAN packet verification (#12571)

This commit addresses several issues in the Nexus verification scripts
to improve test reliability and prevent runtime errors.

- Fixed a TypeError caused by NullField in MLE command checks. When
  p.mle.cmd returns a NullField, it is unhashable and fails when checked
  against a set or list (e.g., 'p.mle.cmd in { ... }'). A truthiness
  check for p.mle.cmd was added to verify_5_6_4.py, verify_5_6_5.py,
  verify_6_2_2.py, and verify_7_1_5.py.

- Improved verify_6_2_2.py robustness:
  - Wrapped must_not_next() checks within 'with pkts.save_index()' to
    ensure the packet iterator remains at the correct position after
    verifying the absence of specific messages.
  - Refactored MLE command filtering into a helper function for better
    readability and consistency.
  - Updated connectivity checks to allow filtering by ROUTER_1_RLOC16
    and specifically look for WPAN Data Requests.

- Cleaned up verify_5_6_4.py by replacing the explicit 'nullField'
  comparison with a more idiomatic truthiness check.

tests/nexus/verify_5_6_4.py

@@ -170,8 +170,7 @@ def verify(pv):
     #     - The Prefix 2 TLV MUST NOT be included
     #   - Active Timestamp TLV
     print("Step 9: Leader (DUT)")
-    pkts.filter(lambda p: p.mle.cmd is not nullField).\
-      filter(lambda p: p.mle.cmd in {consts.MLE_CHILD_UPDATE_REQUEST, consts.MLE_DATA_RESPONSE}).\
+    pkts.filter(lambda p: p.mle.cmd and p.mle.cmd in {consts.MLE_CHILD_UPDATE_REQUEST, consts.MLE_DATA_RESPONSE}).\
       filter(lambda p: p.wpan.src64 == DUT).\
       filter(lambda p: p.wpan.dst64 == SED_1).\
       filter(lambda p: {

tests/nexus/verify_5_6_5.py

@@ -171,7 +171,7 @@ def verify(pv):
     pkts.copy().\
         filter_wpan_src64(LEADER).\
         filter_wpan_dst64(SED_1).\
-        filter(lambda p: p.mle.cmd in {
+        filter(lambda p: p.mle.cmd and p.mle.cmd in {
             consts.MLE_CHILD_UPDATE_REQUEST,
             consts.MLE_DATA_RESPONSE
         }).\

tests/nexus/verify_6_2_2.py

@@ -66,6 +66,10 @@ def verify(pv):
     ROUTER_1 = pv.vars['ROUTER_1']
     ROUTER_2 = pv.vars['ROUTER_2']
 
+    def is_mle_announce_or_child_id_request(p):
+        """Checks if a packet is an MLE Announce or Child ID Request."""
+        return p.mle.cmd and p.mle.cmd in {consts.MLE_ANNOUNCE, consts.MLE_CHILD_ID_REQUEST}
+
     if 'ED_1' in pv.vars:
         DUT = pv.vars['ED_1']
         IS_TOPOLOGY_A = True
@@ -143,10 +147,10 @@ def verify(pv):
             must_next()
 
         # Verify no MLE Announce or additional Child ID Request from DUT
-        pkts.filter_wpan_src64(DUT).\
-            filter(lambda p: hasattr(p, 'mle') and\
-                   p.mle.cmd in [consts.MLE_ANNOUNCE, consts.MLE_CHILD_ID_REQUEST]).\
-            must_not_next()
+        with pkts.save_index():
+            pkts.filter_wpan_src64(DUT).\
+                filter(is_mle_announce_or_child_id_request).\
+                must_not_next()
 
     else:
         # Step 8: SED_1 (DUT) [Topology B only]
@@ -157,16 +161,17 @@ def verify(pv):
         #   - The DUT MUST NOT transmit a MLE Announce message or an additional MLE Child ID Request. If it does, the
         #     test has failed.
         print("Step 8: SED_1 (DUT) [Topology B only]")
-        # We look for any packet from DUT to its parent ROUTER_1 as a sign of connectivity and keep-alive.
+        # We look for a Data Request from DUT to its parent ROUTER_1 as a sign of connectivity and keep-alive.
         pkts.filter_wpan_src64(DUT).\
-            filter_wpan_dst64(ROUTER_1).\
+            filter(lambda p: p.wpan.dst64 == ROUTER_1 or p.wpan.dst16 == pv.vars['ROUTER_1_RLOC16']).\
+            filter_wpan_cmd(consts.WPAN_DATA_REQUEST).\
             must_next()
 
         # Verify no MLE Announce or additional Child ID Request from DUT
-        pkts.filter_wpan_src64(DUT).\
-            filter(lambda p: hasattr(p, 'mle') and\
-                   p.mle.cmd in [consts.MLE_ANNOUNCE, consts.MLE_CHILD_ID_REQUEST]).\
-            must_not_next()
+        with pkts.save_index():
+            pkts.filter_wpan_src64(DUT).\
+                filter(is_mle_announce_or_child_id_request).\
+                must_not_next()
 
     # Step 9: Router_1
     # - Description: To verify connectivity, Harness instructs Router_1 to send an ICMPv6 Echo Request to the DUT link

tests/nexus/verify_7_1_5.py

@@ -188,7 +188,7 @@ def verify(pv):
     # Unicast packets should be decrypted correctly. We allow some leeway for SED poll timing.
     pkts.filter_wpan_src64(ROUTER_1).\
       filter_wpan_dst64(SED_1).\
-      filter(lambda p: p.mle.cmd in {
+      filter(lambda p: p.mle.cmd and p.mle.cmd in {
                                     consts.MLE_CHILD_UPDATE_REQUEST,
                                     consts.MLE_DATA_RESPONSE
                                     }).\