test: add tests for append and store commands

Add comprehensive tests for APPEND and STORE IMAP commands:

STORE command tests (10 new tests):
- Error handling with serverResponseCode
- Filters flags that cannot be used (permanentFlags check)
- Remove operation uses minus prefix
- Returns false when no valid flags for add
- Allows empty flags for set operation (to clear flags)
- Returns false with empty flags for remove
- Default operation is add (+FLAGS)
- With labels uses X-GM-LABELS
- Silent does not apply to labels

APPEND command tests (8 new tests):
- Error handling with serverResponseCode (TRYCREATE)
- Invalid APPENDUID values handling
- NOOP error is caught gracefully
- EXISTS updates mailbox count and emits event
- Does not emit exists when count unchanged
- With both flags and date
- disableBinary prevents literal8

Both commands now have 100% line coverage.

Author: andris9