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/testdata/conformance/server/discover.txtar

@@ -21,6 +21,7 @@ supports, its capabilities, and its identity.
 	"jsonrpc": "2.0",
 	"id": 1,
 	"result": {
+		"resultType": "complete",
 		"ttlMs": 0,
 		"cacheScope": "public",
 		"supportedVersions": [