Add unit test for DELETE /faults status filter verification

[mfaferek93]

Summary

The global DELETE /faults?status= endpoint lacks a test that verifies the status filter actually works (e.g. inject a confirmed fault → DELETE with ?status=pending → verify the confirmed fault still exists). Current integration tests only check that 204 is returned, because demo nodes continuously re-report faults making absence checks flaky.

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Proposed solution (optional)

Add a C++ unit test (~30-50 lines) with a mock fault manager:

1. Mock list_faults() to return known faults with different statuses
2. Call handle_clear_all_faults_global with ?status=pending
3. Verify clear_fault() was only called on pending faults

Existing patterns in test_fault_manager.cpp can be reused — no new test infrastructure needed.

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Additional context (optional)

Raised during PR #228 review by @bburda.