mcp: include resultType on new-protocol responses (#1060)

## Summary

- Add `resultType: "complete"` to non-MRTR server result responses for
2026-07-28 requests.
- Cover `server/discover`, list methods, and completion responses, and
update expected conformance/example output.

## Why

Fixes #1032. The 2026-07-28 schema requires `resultType` on result
objects, but only the MRTR-capable results were setting it.

## Validation

- `go test ./mcp -run
'Test(ServerSessionHandle|CompleteResult|ClientConnectDiscover|InMemory_E2E_Discover)'
-count=1` — passed\n- `go test ./mcp -count=1` — passed\n- `go test
./...` — passed\n- `go vet ./...` — passed

---------

Co-authored-by: ychampion <ychampion@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/protocol_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 )
 
 func TestParamsMeta(t *testing.T) {
@@ -491,7 +492,7 @@ func TestCompleteResult(t *testing.T) {
 			if err := json.Unmarshal([]byte(test.in), &got); err != nil {
 				t.Fatalf("json.Unmarshal(CompleteResult) failed: %v", err)
 			}
-			if diff := cmp.Diff(test.want, got); diff != "" {
+			if diff := cmp.Diff(test.want, got, cmpopts.IgnoreUnexported(CompleteResult{})); diff != "" {
 				t.Errorf("CompleteResult unmarshal mismatch (-want +got):\n%s", diff)
 			}
 		})