Add an option to only summarize last email for thread summaries

Is your feature request related to a problem? Please describe.

Hi team,
It would be great to have an option (like a toggle button or something like that) to only send the last email to the AI for the "Thread summary" feature. The reason is that normally the other emails are already included in the body of the last email, so it's a waste of tokens and time sending/processing the same information a lot of times (once per email in the thread).

Describe the solution you'd like

Add a button in Mail's configuration that enables code like:

Instead of the $messagesBodies in
https://github.com/nextcloud/mail/blob/203ce3603f6abf3640dbc4856e422162a55e5531/lib/Service/AiIntegrations/AiIntegrationsService.php#L146

Have only the body of the last one:

$lastMessage = $messages[array_key_last($messages)];
$mailbox = $this->mailManager->getMailbox($currentUserId, $lastMessage->getMailboxId());
$imapMessage = $this->mailManager->getImapMessage(
    $client,
    $account,
    $mailbox,
    $lastMessage->getUid(), true
);
$messagesBodies = [$imapMessage->getPlainBody()];

Describe alternatives you've considered

No response

Additional context

No response

Start with lib/Service/AiIntegrations/AiIntegrationsService.php around line 146 and trace how thread summaries assemble message bodies. Then inspect Mail's configuration entry points for where the option should be exposed. Done means the setting can enable sending only the last email's plain body to the AI, while the existing behavior remains available.