mcp: move to 2025-06-18 schema (#8)

* mcp: move to 2025-06-18 schema

Add types and fields for the latest MCP schema.

The main challenge here was dealing with the presence of _meta fields
on many types, like Tool, Prompt and Resource.

- Distinguish between the _meta field of a request's params, which
  includes a progress token, from the _meta field on other types,
  which is simply a map.

- Redefine GetMeta to return a pointer to the map. We wanted to avoid
  that, but I don't see a clean way to do so. An alternative would be
  to have GetMeta return a map, and SetMeta to set it.

- Provide GetProgressToken for requests only (not notifications or other
  types).

- Still to do: deal with the meta and annotation fields of ContentBlock,
  which are currently duplicated for Resource and ResourceContents.

* Meta and progress token redesign

mcp/tool.go

@@ -132,7 +132,7 @@ func newRawHandler(st *ServerTool) rawToolHandler {
 		// rather than returned as jsonrpc2 server errors.
 		if err != nil {
 			return &CallToolResult{
-				Content: []*Content{NewTextContent(err.Error())},
+				Content: []*ContentBlock{NewTextContent(err.Error())},
 				IsError: true,
 			}, nil
 		}