## Summary
- allow ISO 8601 normalization callers to provide an explicit fallback
while preserving the existing default behavior
- use the extraction conversation time when `valid_at` is missing or
invalid
- clear an invalid optional `invalid_at` instead of writing an
unparseable value
- include the rejected timestamp value in the error log
This addresses the timestamp write-through portion of #18415. It is
intentionally separate from #18462, which fixes the semantic output
prompt.
Co-authored-by: Loong <184861530+yzl0ng@users.noreply.github.com>
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