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

examples/hello/main.go

@@ -24,7 +24,7 @@ type HiArgs struct {
 
 func SayHi(ctx context.Context, ss *mcp.ServerSession, params *mcp.CallToolParamsFor[HiArgs]) (*mcp.CallToolResultFor[struct{}], error) {
 	return &mcp.CallToolResultFor[struct{}]{
-		Content: []*mcp.Content{
+		Content: []*mcp.ContentBlock{
 			mcp.NewTextContent("Hi " + params.Name),
 		},
 	}, nil

examples/sse/main.go

@@ -21,7 +21,7 @@ type SayHiParams struct {
 
 func SayHi(ctx context.Context, cc *mcp.ServerSession, params *mcp.CallToolParamsFor[SayHiParams]) (*mcp.CallToolResultFor[any], error) {
 	return &mcp.CallToolResultFor[any]{
-		Content: []*mcp.Content{
+		Content: []*mcp.ContentBlock{
 			mcp.NewTextContent("Hi " + params.Name),
 		},
 	}, nil