Add N9300 switch memory check

[Priyanka-Patil14]

Summary

This PR adds a new validation check: N9300 Switch Memory.

The check runs only when the target upgrade version is 6.1 and validates that N9300-series switches have at least 24 GB memory before upgrade.

What Changed

• Added n9300_switch_memory_check in aci-preupgrade-validation-script.py
• Added validation documentation in docs/docs/validations.md
• Added dedicated unit tests and test data under:
  • tests/checks/n9300_switch_memory_24g_check/

Check Behavior

• Returns MANUAL if target version is missing
• Returns N/A if target version is not 6.1
• Returns N/A if no N9300 switches are present
• Returns FAIL_O for N9300 nodes with memory < 24 GB
• Returns PASS when all applicable N9300 nodes have >= 24 GB

##Test Validation
Executed:
pytest tests/checks/n9300_switch_memory_24g_check/test_n9300_switch_memory_24g_check.py -q

Result:
7 passed in 0.11s

No failures observed.