This commit adds support for IMAP servers that don't provide UIDVALIDITY,
such as Tencent Enterprise Mail (腾讯企业邮箱).
Changes:
- Added `generate_synthetic_uidvalidity()` function that creates a stable
hash-based UIDVALIDITY from the mailbox name
- Modified `reconcile_mailboxes()` to use synthetic UIDVALIDITY when the
server doesn't provide one
- Servers without UIDVALIDITY can now sync all mailboxes including system
folders (Sent Messages, Drafts, Deleted Messages)
- Incremental sync is supported via the synthetic UIDVALIDITY
- Added warning logs to indicate when synthetic UIDVALIDITY is in use
- Updated mailbox metadata to store the resolved UIDVALIDITY
Fixes issues with:
- Tencent Enterprise Mail (腾讯企业邮箱)
- Other non-compliant IMAP servers
- Mailboxes that don't properly support UIDVALIDITY"
