bug(imap): APPEND response hardcodes UIDVALIDITY=1 in APPENDUID code

[ssilvius]

Problem

`handleAppend` in `packages/imap/src/commands/extensions.ts` hardcodes `1` as the UIDVALIDITY value in the `APPENDUID` response code:

```typescript
// RFC 4315: APPENDUID response code
return [formatTagged(tag, "OK", `[APPENDUID 1 ${result.uid}] APPEND completed`)];
```

Per RFC 4315 Section 3, the `APPENDUID` response code is `APPENDUID `. The uidValidity must be the current mailbox UIDVALIDITY, not a constant. Clients correlate their local UID cache against the (uidValidity, uid) tuple -- if uidValidity changes and the client sees a stale `1`, it may believe its cache is still valid when it is not, or it may invalidate incorrectly when an unrelated mailbox swaps in.

Fix scope

1. `handleAppend` needs access to the target mailbox's `uidValidity`. The mailbox adapter already returns `uidValidity` from `getFolderStats`; the APPEND handler should fetch it (or receive it as a parameter) before emitting the response.
2. Update the response line to use the real uidValidity
3. Add a test referencing RFC 4315 Section 3 that asserts APPENDUID uses the actual mailbox uidValidity across mailboxes with different UIDVALIDITY values.

Related

• Discovered during the doc accuracy audit pass on PR docs: fix interface drift across adapters.md, better-auth-resend, react-email #81.
• The doc example in `packages/imap/docs/commands.md` shows `APPENDUID 1 305` which happens to match the current buggy behavior, so no doc change needed -- just fix the implementation.