C1: build_ping_embed set sender_id field value="" whenever sender_id was
None, which manual_share.py always passes -- Discord rejects empty embed
field values (min length 1), so every manual-share ping failed to send.
Fixed by using a zero-width space sentinel that round-trips back to ""
(raw_content) or None (sender_id) in parse_ping_embed. Added regression
tests asserting no field value is ever empty after build_ping_embed runs.
C2: raw_content over 1024 chars (Discord's embed field cap) caused
inbox.send() to raise before the UID was marked seen, so oversized emails
were retried forever and never ingested. Added a defensive hard truncation
in build_ping_embed itself, plus an explicit truncation in email_ingest.py
before the embed is built (lossy for very long emails, acceptable for MVP).
- 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.
