- Remove 'or (empty)' fallback in build_ping_embed to preserve empty strings in raw_content
- Fix parse_ping_embed to use direct indexing for all required fields (modality, raw_content, captured_at)
- Remove unused timezone import
- Add test case for empty raw_content round-trip to catch future regressions
Fixes code review findings: empty raw_content was being converted to literal "(empty)" string,
breaking lossless round-trip requirement needed by Task 7's thread-open handler.
