Rir assignment check hardening

[vegu]

Added

• RdapBootstrapError (subclass of RdapNotFoundError) raised on a self-bootstrap lookup miss, so callers can distinguish "no registry was queried" from an authoritative 404

Fixed

• RIRAssignmentLookup: validate downloaded delegated-stats data (HTTP status + completeness against the file's summary count) and never overwrite a good cache with a failed/truncated fetch, so a partial download no longer makes allocated ASNs read as unassigned
• RIRAssignmentLookup: cache freshness now uses elapsed seconds instead of floored timedelta.days, so cache_days=1 refreshes at 24h rather than ~48h
• RIRAssignmentLookup.load_data: a load that fails partway no longer memoizes partial state on the instance, so a retry re-attempts all downloads instead of silently serving a subset of RIRs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.14%. Comparing base (e8e35f2) to head (de3a07e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   87.98%   89.14%   +1.16%     
==========================================
  Files          20       20              
  Lines        1182     1244      +62     
==========================================
+ Hits         1040     1109      +69     
+ Misses        142      135       -7     

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