Return structured citations to clients that advertise citation rendering capability, following draft SEP-3094 format

[karth295]

What changed

Clients that advertise that they support rendering structured citations will get back structured citations, rather than having citation metadata dumped into the markdown response. There is no change for clients that do not opt in.

See modelcontextprotocol/modelcontextprotocol#3094 for more details on the draft spec.

Before:

{
  "content": [
    {
      "type": "text",
      "text": "Supported claim[1].\n\nCitations:\n[1] https://example.com/source\n"
    }
  ]
}

After:

{
  "content": [
    {
      "type": "text",
      "text": "Supported claim[1]."
    }
  ],
  "citations": [
    {
      "id": "1",
      "name": "Source",
      "url": "https://example.com/source#:~:text=Supporting%20passage",
      "text": "Supporting passage"
    }
  ]
}