Added 5 new tests and improved functionality a bit.

It seems cleaner this way.

- Added a func comment
- Removed unused arguments in imap package
- Fixed grammar in ROADMAP

Was duplicated 5x

- Handle multiple whitespace chars
- Cover edge-cases around whitespaces with tests

These types had no names before, but they were used at a few places, nicer this way.

Extracted it to a WriteJSONResponse helper

Naming is hard.

- Add concurrent access and edge case tests for pool
- Add tests for fetch.go and processIncrementalMessage
- Fix unchecked error in FetchFullMessage
- Export GetListenerConnection with ListenerClient interface
- Fix Close() deadlock by using TryLock() for graceful shutdown
- Remove all FIXME-TEST comments

- Use errors.Is() with sentinel error for invalid search queries
- Extract duplicated pagination limit logic to helpers.go

- Refactor the API to expose release explicitly and remove the artificial minimum hold time of 5 sec. → more performant pool with the same number of workers
- Share a single *imap.Pool between FoldersHandler and Service. → less workers needed
- Make maxWorkers configurable. → Their number can be high in tests, conservative in prod
- Clean up dbPool vs. imapPool naming.

For more safety to get worker slots released.

- Add VMAIL_ENV=test to avoid "Warning: .env file not found, using environment variables" logs.
- Search: Treat client cancellations as non-errors to avoid "SearchHandler: Failed to search: failed to get IMAP client: failed to get user settings: failed to get user settings: context canceled" logs.
- Require THREAD support from prod IMAP servers, so we don't need this error message: "THREAD command not supported, falling back to SEARCH: THREAD command returned error: Unknown command".