❤️ TEST-#38: Add coverage for Sync and Restore single-mailbox paths

[FernandoCelmer]

Summary

New test modules:

• tests/clients/imap/test_sync.py — 5 tests for Sync.sync:
  • inserts new messages
  • skips duplicate message-ids on second pass
  • empty mailbox = no work
  • progress callback invoked, ends at total
  • storage save failure recorded in result.errors
• tests/clients/imap/test_restore.py — 7 tests for Restore.restore_mailbox + helpers:
  • uploads new message
  • skips messages already on server (via _server_message_ids)
  • missing storage entry is dropped, no APPEND
  • skip_duplicates=False forces upload
  • _ensure_mailbox: existing mailbox is no-op, missing mailbox is created
  • _server_message_ids: empty SEARCH returns empty set

Total mocked IMAP scenarios use the existing make_fake_client fixture, so no new dependencies.

Closes #38

Test plan

• pytest tests/clients/imap/test_sync.py tests/clients/imap/test_restore.py — 12 passed

[FernandoCelmer]

🔍 Code Review

Code issues found: 0

Test-only PR. Two new modules close the biggest test-coverage gap in the library:

test_sync.py — covers insert, dedupe-on-second-pass, empty mailbox, progress callback invariant (ends at total), and the storage-error path that lands in result.errors.

test_restore.py — covers upload, server-side dedupe via _server_message_ids, missing-storage-entry drop, skip_duplicates=False force-upload, and _ensure_mailbox create/no-op branches.

Reusing make_fake_client keeps the suite hermetic — no new deps. Combined with #81 this fills the two open test-coverage tickets in one round.

Closes #38.