utils: do not report invalid IPv6 ranges as overlap

[tllovesxs]

Summary

• Return false for malformed or reversed IPv6 ranges instead of reporting them as overlapping.
• Log the offending range values so invalid input is visible to operators.
• Catch only IllegalArgumentException; unexpected runtime failures are no longer silently swallowed.
• Add regression coverage for overlap, disjoint ranges, malformed input, and reversed ranges.

Fixes #32.

Test plan

• git diff --check
• Added TestIPv6NetworkUtilsEdgeCases.
• Confirmed the underlying IPv6 library rejects reversed ranges.
• The helper test passes after the narrowed exception handling.

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