fix: correct sentinel test timestamps and four-segment detection test
Fix timestamp values in format_rfc3339_utc_pins_known_points test to use
correct UNIX timestamps for leap days (1972, 2000, 2024). The previously
hardcoded values were off by a day or more.
Update decode_compact_malformed_four_segments test to expect
JwtError::MalformedCompact (which now correctly catches 4+ segments) instead
of Base64Decode error (which was the old behavior when splitn(3, '.') failed
to detect the extra segment).
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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