feat(telegram): send multiple outbound images as albums

When an agent sends several images in one response, the Telegram channel calls send_photo once per item. Telegram shows them as separate messages instead of an album.

Use sendMediaGroup for compatible outbound media lists:

• Group consecutive photos and videos into albums of 2–10 items.
• Preserve the current individual-send behavior for single items and unsupported/mixed attachment types.
• Validate every remote URL before sending.
• If an album request fails, fall back to individual sends so attachments are not lost.
• Keep the reply text as a separate message for now; an album only supports one caption.

Add Telegram channel tests for local files, remote URLs, batches over ten, and fallback behavior.

Related: #6123 tracks remote image URLs with query strings being classified as documents instead of photos, which prevents otherwise compatible remote images from being grouped.