Changes:
- CheckStaleness now returns errors for corrupted last_import_time metadata
instead of silently returning false (bd-o4qy)
- Added handling for empty string metadata (memory store behavior)
- Enhanced warning messages when staleness check fails to be more explicit
that operation continues with potentially stale data (bd-n4td)
- Added test coverage for corrupted metadata scenario
Closes bd-2q6d, bd-o4qy, bd-n4td
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Co-Authored-By: Claude <noreply@anthropic.com>
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