[nexus] refactor test 1.2.LP.5.3.1 and remove kAsSsed mode (#12615)

This commit refactors Nexus test 1.2.LP.5.3.1 to better align with
the specification and improves the robustness of the test logic and
verification script. It also removes the kAsSsed join mode to favor
explicit CSL configuration in tests.

Changes:
- Removed kAsSsed from Nexus::Node::JoinMode and Node::Join() to
  encourage tests to manage CSL parameters explicitly.
- Updated test_1_2_LP_5_3_1.cpp to join as a regular SED first and
  enable CSL after attachment. This matches the specification's
  requirement to establish synchronization via Child Update Request.
- Introduced constants in test_1_2_LP_5_3_1.cpp for CSL period,
  synchronization time, and other parameters to avoid magic numbers.
- Enhanced verify_1_2_LP_5_3_1.py with comprehensive checks for:
    - MLE Child Update Request/Response exchange.
    - IEEE 802.15.4-2015 frame versions.
    - ICMPv6 Echo Request/Response forwarding through the Leader.
    - Absence of MAC Data Requests after CSL synchronization.
- Fixed an issue in verify_1_2_LP_5_3_1.py where packet numbers
  were not correctly handled in pkts.range().

tests/nexus/platform/nexus_node.cpp

@@ -87,15 +87,6 @@ void Node::Join(Node &aNode, JoinMode aJoinMode)
         break;
     case kAsSed:
         break;
-    case kAsSsed:
-    {
-        static constexpr uint32_t kDefaultCslPeriod  = 100 * 1000 / OT_US_PER_TEN_SYMBOLS; // 100 ms
-        static constexpr uint32_t kDefaultCslTimeout = 30;                                 // 30 seconds
-
-        Get<Mac::Mac>().SetCslPeriod(kDefaultCslPeriod);
-        Get<Mle::Mle>().SetCslTimeout(kDefaultCslTimeout);
-        break;
-    }
     }
 
     SuccessOrQuit(Get<Mle::Mle>().SetDeviceMode(Mle::DeviceMode(mode)));