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/streamable_test.go

@@ -82,7 +82,7 @@ func TestStreamableTransports(t *testing.T) {
 
 	// 5. Verify that the correct response is received.
 	want := &CallToolResult{
-		Content: []*Content{{Type: "text", Text: "hi streamy"}},
+		Content: []*ContentBlock{{Type: "text", Text: "hi streamy"}},
 	}
 	if diff := cmp.Diff(want, got); diff != "" {
 		t.Errorf("CallTool() returned unexpected content (-want +got):\n%s", diff)
@@ -583,4 +583,3 @@ func TestEventID(t *testing.T) {
 		})
 	}
 }
-