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

Fixes #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]

Integration run summary (pipeline 202, sanitized):

• Python 2.7 and 3.8 unit-test jobs passed.
• The RTC battery/F2421 validation executed successfully in 110 captured fabric runs: all reported passed, with no failure detail.
• Every corresponding F2421 API query returned zero matching active faults, so this run validates the APIC-compatible query and clean-fabric path.
• The active-fault FAIL - OUTAGE WARNING!! path remains covered by the focused unit test (5 passed); it was not exercised because no test fabric reported F2421.
• The seven integration jobs ended failed due to pre-existing validation findings in the test environment; the RTC validation did not produce any of those findings.

Individual fabric identifiers and artifact locations intentionally omitted.