fix(api): document image_id parsing for hyphenated thumbnail keys (#15115) (#15116)

### What problem does this PR solve?

Fixes #15115.

`GET /api/v1/documents/images/<image_id>` returned **Image not found**
when the thumbnail storage object key contained hyphens (e.g.
`page-1.png`). Document APIs build URLs as `{dataset_id}-{thumbnail}`,
but `get_document_image()` used `image_id.split("-")` and required
exactly two segments, so keys like `<kb_id>-page-1.png` were rejected
even though the blob existed.

This PR splits only on the first hyphen (`split("-", 1)`) and sets
`Content-Type` from the object key extension via `CONTENT_TYPE_MAP`
instead of hardcoding `image/JPEG`.