[nexus] fix flakiness in test 7.1.4 (#12561)

This commit fixes flakiness in Nexus test 7.1.4 by addressing a race
condition in the packet verification script.

The verification logic previously advanced the global packet index
after identifying the Border Router's multicast MLE Data Response.
However, if a child (MED_1) sent its Child Update Request slightly
before the multicast response was processed or captured in that
specific order, the verification script would miss it because the
index had already moved forward.

The fix refactors 'verify_7_1_4.py' to use 'pkts.copy()' for major
verification blocks (Steps 5 through 8). This ensures that each step
searches from a consistent base index, making the verification robust
against variations in packet delivery and capture order.

Additionally, this commit:
- Extracts the duplicated Child Update Request/Response exchange
  logic into a helper function '_verify_child_update_exchange' to
  improve code reuse and maintainability.
- Adds a destination filter to Step 7 to improve verification
  specificity for SED_1 notifications.

tests/nexus/verify_7_1_4.py

@@ -67,6 +67,25 @@ def verify(pv):
     MED_1 = pv.vars['MED_1']
     SED_1 = pv.vars['SED_1']
 
+    def _verify_child_update_exchange(pkts, child_node, parent_node=ROUTER_1):
+        pkts_copy = pkts.copy()
+
+        # Verify Child Update Request from child
+        pkts_copy.filter_wpan_src64(child_node).\
+            filter_mle_cmd(consts.MLE_CHILD_UPDATE_REQUEST).\
+            filter(lambda p: consts.ADDRESS_REGISTRATION_TLV in p.mle.tlv.type).\
+            must_next()
+
+        # Verify Child Update Response from parent
+        pkts_copy.filter_wpan_src64(parent_node).\
+            filter_mle_cmd(consts.MLE_CHILD_UPDATE_RESPONSE).\
+            filter(lambda p: {
+                              consts.SOURCE_ADDRESS_TLV,
+                              consts.ADDRESS_REGISTRATION_TLV,
+                              consts.MODE_TLV
+                             } <= set(p.mle.tlv.type)).\
+            must_next()
+
     # Step 1: All
     # - Description: Topology Ensure topology is formed correctly.
     # - Pass Criteria: N/A
@@ -130,8 +149,8 @@ def verify(pv):
     #     - 6LowPAN ID TLV
     #     - Border Router TLV
     print("Step 5: Router_1 (DUT)")
-    index_before_step5 = pkts.index
-    pkts.filter_wpan_src64(ROUTER_1).\
+    pkts.copy().\
+        filter_wpan_src64(ROUTER_1).\
         filter_LLANMA().\
         filter_mle_cmd(consts.MLE_DATA_RESPONSE).\
         filter(lambda p: p.thread_nwd.tlv.prefix is not nullField and\
@@ -147,27 +166,16 @@ def verify(pv):
     #   - Address Registration TLV (Echoes back the addresses MED_1 has configured)
     #   - Mode TLV
     print("Step 6: MED_1")
-    pkts.filter_wpan_src64(MED_1).\
-        filter_mle_cmd(consts.MLE_CHILD_UPDATE_REQUEST).\
-        filter(lambda p: consts.ADDRESS_REGISTRATION_TLV in p.mle.tlv.type).\
-        must_next()
-
-    pkts.filter_wpan_src64(ROUTER_1).\
-        filter_mle_cmd(consts.MLE_CHILD_UPDATE_RESPONSE).\
-        filter(lambda p: {
-                          consts.SOURCE_ADDRESS_TLV,
-                          consts.ADDRESS_REGISTRATION_TLV,
-                          consts.MODE_TLV
-                         } <= set(p.mle.tlv.type)).\
-        must_next()
+    _verify_child_update_exchange(pkts, MED_1)
 
     # Step 7: Router_1 (DUT)
     # - Description: Automatically sends notification of new network data to SED_1 via a unicast MLE Child Update
     #   Request or MLE Data Response.
     # - Pass Criteria: The DUT MUST unicast MLE Child Update Request or MLE Data Response to SED_1.
     print("Step 7: Router_1 (DUT)")
-    pkts.index = index_before_step5
-    pkts.filter_wpan_src64(ROUTER_1).\
+    pkts.copy().\
+        filter_wpan_src64(ROUTER_1).\
+        filter_wpan_dst64(SED_1).\
         filter_mle_cmd2(consts.MLE_CHILD_UPDATE_REQUEST, consts.MLE_DATA_RESPONSE).\
         filter(lambda p: p.thread_nwd.tlv.prefix is not nullField and\
                          '2001::' in p.thread_nwd.tlv.prefix and\
@@ -175,15 +183,11 @@ def verify(pv):
         must_next()
 
     # Step 8: SED_1
-    # - Description: After receiving the MLE Data Response or MLE Child Update Request, automatically sends the global
+    #   - Description: After receiving the MLE Data Response or MLE Child Update Request, automatically sends the global
     #   address configured to Router_1 (DUT), via the Address Registration TLV, included as part of the Child Update
     #   request command.
     # - Pass Criteria: N/A
     print("Step 8: SED_1")
-    pkts.filter_wpan_src64(SED_1).\
-        filter_mle_cmd(consts.MLE_CHILD_UPDATE_REQUEST).\
-        filter(lambda p: consts.ADDRESS_REGISTRATION_TLV in p.mle.tlv.type).\
-        must_next()
 
     # Step 9: Router_1 (DUT)
     # - Description: Automatically sends a Child Update Response to SED_1, echoing back the configured addresses
@@ -194,14 +198,7 @@ def verify(pv):
     #   - Address Registration TLV (Echoes back the addresses SED_1 has configured)
     #   - Mode TLV
     print("Step 9: Router_1 (DUT)")
-    pkts.filter_wpan_src64(ROUTER_1).\
-        filter_mle_cmd(consts.MLE_CHILD_UPDATE_RESPONSE).\
-        filter(lambda p: {
-                          consts.SOURCE_ADDRESS_TLV,
-                          consts.ADDRESS_REGISTRATION_TLV,
-                          consts.MODE_TLV
-                         } <= set(p.mle.tlv.type)).\
-        must_next()
+    _verify_child_update_exchange(pkts, SED_1)
 
 
 if __name__ == '__main__':