[nexus] implement test 1.2.LP.7.2.2 for Forward Series Probing (#12626)

This commit implements Nexus test 1.2.LP.7.2.2 to verify that the
DUT (Leader) can successfully support a minimum of 6 simultaneous
children (3 SEDs and 3 SSEDs) performing link metrics operations.

Specifically, the test verifies that:
- The DUT correctly handles simultaneous Forward Series Link Metrics
  Management Requests and Enhanced ACK probing requests.
- The DUT properly aggregates metrics for multiple concurrent series
  with different configurations (MAC Data Requests, Link Layer data
  frames, LQI, RSSI, and Link Margin).
- Aggregated results are accurately reported in MLE Data Responses
  when queried by multiple children.
- The DUT successfully provides link metrics in Enhanced ACKs for
  both SED and SSED children.

Changes:
- Added test_1_2_LP_7_2_2.cpp to implement the test logic, including
  node configuration, management requests, and data collection.
- Added verify_1_2_LP_7_2_2.py for automated PCAP verification of
  MLE TLVs and Enhanced ACK Vendor IEs.
- Updated verify_utils.py to include necessary layer fields for
  Link Metrics verification in pktverify.
- Registered the new test in CMakeLists.txt and run_nexus_tests.sh.