test: add tests for create and enable commands

Add comprehensive tests for CREATE and ENABLE IMAP commands:

CREATE command tests (10 tests):
- State validation (unauthenticated, selected states)
- Successful mailbox creation with auto-subscribe
- MAILBOXID extraction from OBJECTID extension
- Path normalization with empty path
- ALREADYEXISTS error handling (returns created: false)
- General error propagation

ENABLE command tests (13 tests):
- Capability and state validation
- Single and multiple extension enabling
- Filtering of unsupported extensions
- Uppercase conversion of extension names
- Empty and null response handling
- connection.enabled Set updates

Both commands now have 100% code coverage.

Author: andris9