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