faults: Add F2421 RTC battery validation

[monrog2]

Summary

• add an upgrade-blocking check for active F2421 faults reporting the exact The RTC battery voltage is low reason
• scope the APIC faultInst query by both fault code and RTC-low description, then enforce exact reason matching client-side
• report affected pod, node, supervisor, severity, and lifecycle while preserving malformed active rows
• document remediation and add issue-derived coverage for active, stale, unrelated-reason, empty, and malformed responses

Validation

• python3 -m py_compile aci-preupgrade-validation-script.py tests/checks/rtc_battery_voltage_low_check/test_rtc_battery_voltage_low_check.py
• git diff --check
• focused tests: 5 passed
• full suite: 921 passed

Closes #351

[monrog2]

Blocking feedback: the RTC battery fault is detected correctly, but its result category understates the documented operational impact. GitHub does not allow the PR author to submit a Request changes review on their own PR.

[monrog2]

Blocking: Please classify this as FAIL_O rather than FAIL_UF and update the active/malformed-fault test expectations accordingly. Issue #351 and the new validation documentation state that a low RTC battery can reset the switch clock during an upgrade or power cycle, causing certificate validation to fail and preventing the switch from rejoining the fabric. That is an outage risk, so FAIL - OUTAGE WARNING!! is the appropriate result category.