gmail: retry messages after body fetch failures

[fallintoplace]

Problem

Gmail search and body retrieval happen separately. When gog gmail get failed or timed out, the source still created a record from the search metadata with an empty body and advanced the shared timestamp cursor. The recovered message was then behind the cursor, leaving only the header-only record permanently stored.

Fix

• represent an unsuccessful body retrieval separately from a valid empty body
• skip records whose body could not be retrieved
• keep the previous cursor when any message in the batch fails
• continue returning successfully retrieved messages and rely on inbox ID deduplication for unchanged retries

Verification

Regression coverage exercises a partial failure followed by recovery, an exception during initial sync, and a nonzero body-fetch command.

• .venv/bin/python -m pytest -q tests/test_gmail_source.py tests/test_source_filters.py tests/test_db.py: 134 passed
• broader suite: 2941 passed

[alex-clickhouse]

One issue with this approach: suppose there is a message that for some reason fails gog gmail get deterministically. Then the cursor is going to be stuck at that point, until it falls out of the batch size window (and, since any failure means no cursor progress, it would take a bunch of other messages with it).

One option would be to retry in place for 2-3 times with exponential backoff. That still doesn't save us from longer-lived failures though.

More complex would be to do something similar to the Telegram one, which stores json, and then we could keep track of individual ids and their status. But that's probably a bit too much.