query_documents should say when a result matched in an attachment and name the file

Highlighting in query_documents only runs on the rendered content fields. When a hit comes from attachment text or an object property the result shows no snippet, so the agent cannot tell why the page matched and has to open the page, read the Attachments header line and guess which file to fetch with get_attachment.

Extend highlighting to attcontent, filename and objcontent, and render the snippet with its origin, for example "Matched in attachment report.pdf: ...". When the match is in attachment text, include the attachment reference the agent can pass straight to get_attachment.

Acceptance:

• A result matched only through attachment text shows a snippet from that text and the file name.
• A result matched only through an object property shows a snippet from that property.
• Content matches keep today's snippet format.
• Attachment references in the output follow the same escaping rules as the Attachments line of get_document.

Depends on the query fields being extended first (see the linked issue).