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).
_extract_body previously discarded real HTML content, replacing it with
a placeholder whenever a multipart email had no text/plain part. Now
falls back to the raw text/html payload before giving up, only using
the placeholder when a message has no text-bearing part at all.
poll_mailboxes also had no error isolation: an unhandled IMAP exception
from any one mailbox would stop the discord.ext.tasks.Loop permanently,
silently killing ingestion for all three mailboxes. Each mailbox's poll
is now wrapped in try/except so one failure doesn't block the others.
