Root cause: flutter test ran all 3 integration test files in parallel
against the same Stalwart instance. Concurrent SMTP/IMAP from
email_repository_imap_test and concurrent_sync_test caused SMTP rate
limiting (4th send hung for ~27s) and flushPendingChanges race failures.
Fixes:
- stalwart-dev/test.sh: add --concurrency=1 so test files run serially
- concurrent_sync_test: reduce timeout 2 min → 30 s (tests now pass in ~2s)
- imap_client_factory + test helpers: set defaultResponseTimeout=20s on
ImapClient so individual IMAP commands never block indefinitely
- jmap_client: reduce HTTP call timeout 30 s → 10 s (local server; keeps
stacked-timeout total well below any reasonable per-test limit)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
