fix(server): match IMAP \Noselect attribute case-insensitively (#20043)

## Summary

`ImapGetAllFoldersService.isMailboxSelectable` checked
`mailbox.flags?.has('\\Noselect')`, which is case-sensitive. Per [RFC
3501 §6.3.8](https://www.rfc-editor.org/rfc/rfc3501#section-6.3.8), IMAP
attribute names are case-insensitive — different servers spell the flag
differently:

| Server | Spelling |
|---|---|
| Dovecot | `\Noselect` |
| Stalwart | `\NoSelect` |
| Cyrus | `\Noselect` |
| RFC examples | `\NOSELECT` |

The previous check only caught Dovecot's spelling. On other servers,
virtual namespace placeholders (e.g. Stalwart's `Shared Folders` parent)
passed through `isMailboxSelectable` and got persisted as folders. When
`MessagingMessageListFetchJob` later ran, it issued `SELECT "Shared
Folders"`, the server correctly rejected it with `NO [NONEXISTENT]
Mailbox does not exist.`, and the entire message-list fetch failed for
the channel.

## Reproduction

1. Connect an IMAP account whose server advertises a `\NoSelect` (or
`\NOSELECT`) namespace placeholder. Stalwart Mail v0.15.x exhibits this
with shared mailboxes:
   ```
   * LIST (\NoSelect) "/" "Shared Folders"
   ```
2. The folder discovery job persists it as a syncable folder.
3. `MessagingMessageListFetchJob` runs and fails:
   ```
   IMAP: Error fetching message list: D0 SELECT "Shared Folders"
   responseStatus: NO  serverResponseCode: NONEXISTENT
   ```

## Fix

Iterate the flag set and lowercase-compare against `'\\noselect'`. This
matches every legal spelling without changing behavior for compliant
`\Noselect` clients.

```diff
 private isMailboxSelectable(mailbox: ListResponse): boolean {
-  return !mailbox.flags?.has('\\Noselect');
+  if (!mailbox.flags) return true;
+  for (const flag of mailbox.flags) {
+    if (flag.toLowerCase() === '\\noselect') return false;
+  }
+  return true;
 }
```

## Test plan

- [x] Existing `\Noselect` test cases (`should not issue STATUS against
a \Noselect folder`, parent-reference preservation, Sent-folder
exclusion) still pass — the lowercase comparison subsumes them.
- [x] New parameterized test covers `\Noselect`, `\NoSelect`,
`\NOSELECT`, `\noselect` spellings against a Stalwart-style `Shared
Folders` namespace placeholder, asserting Twenty does **not** issue
`STATUS` on the placeholder and does **not** include it in the
discovered folder set.

---------

Co-authored-by: neo773 <62795688+neo773@users.noreply.github.com>

packages/twenty-server/src/modules/messaging/message-folder-manager/drivers/imap/services/__tests__/imap-get-all-folders.service.spec.ts

@@ -190,5 +190,46 @@ describe('ImapGetAllFoldersService', () => {
       );
       expect(result.find((f) => f.isSentFolder)).toBeUndefined();
     });
+
+    it.each([['\\Noselect'], ['\\NoSelect'], ['\\NOSELECT'], ['\\noselect']])(
+      'should treat %s as non-selectable (RFC 3501 attribute names are case-insensitive)',
+      async (flagSpelling) => {
+        const mailboxList = [
+          createMockMailbox({ path: 'INBOX' }),
+          createMockMailbox({
+            path: 'Shared Folders',
+            flags: new Set([flagSpelling]),
+          }),
+          createMockMailbox({
+            path: 'Shared Folders/team/INBOX',
+            name: 'INBOX',
+            parentPath: 'Shared Folders/team',
+          }),
+        ];
+
+        mockImapClient.list.mockResolvedValue(mailboxList);
+        mockImapClient.status.mockImplementation(async (path: string) => {
+          if (path === 'Shared Folders') {
+            throw new Error(`Mailbox doesn't exist: ${path}`);
+          }
+
+          return { uidValidity: BigInt(1) } as any;
+        });
+
+        const result = await service.getAllMessageFolders(
+          CONNECTED_ACCOUNT,
+          MESSAGE_CHANNEL,
+        );
+
+        expect(mockImapClient.status).not.toHaveBeenCalledWith(
+          'Shared Folders',
+          expect.anything(),
+        );
+
+        const paths = result.map((f) => f.externalId?.split(':')[0]);
+
+        expect(paths).not.toContain('Shared Folders');
+      },
+    );
   });
 });

packages/twenty-server/src/modules/messaging/message-folder-manager/drivers/imap/services/imap-get-all-folders.service.ts

@@ -138,7 +138,21 @@ export class ImapGetAllFoldersService implements MessageFolderDriver {
   }
 
   private isMailboxSelectable(mailbox: ListResponse): boolean {
-    return !mailbox.flags?.has('\\Noselect');
+    // Per RFC 3501, IMAP attribute names are case-insensitive. Different
+    // servers vary the spelling (Dovecot: \Noselect, Stalwart: \NoSelect),
+    // so we compare lowercased to avoid attempting SELECT on a virtual
+    // namespace placeholder, which the server would reject as NONEXISTENT.
+    if (!mailbox.flags) {
+      return true;
+    }
+
+    for (const flag of mailbox.flags) {
+      if (flag.toLowerCase() === '\\noselect') {
+        return false;
+      }
+    }
+
+    return true;
   }
 
   private isValidMailbox(