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/transport_example_test.go

@@ -42,7 +42,7 @@ func ExampleLoggingTransport() {
 	}
 
 	// Output:
-	// read: {"jsonrpc":"2.0","id":1,"result":{"ttlMs":0,"cacheScope":"public","supportedVersions":["2026-07-28","2025-11-25","2025-06-18","2025-03-26","2024-11-05"],"capabilities":{"logging":{}},"serverInfo":{"name":"server","version":"v0.0.1"}}}
+	// read: {"jsonrpc":"2.0","id":1,"result":{"resultType":"complete","ttlMs":0,"cacheScope":"public","supportedVersions":["2026-07-28","2025-11-25","2025-06-18","2025-03-26","2024-11-05"],"capabilities":{"logging":{}},"serverInfo":{"name":"server","version":"v0.0.1"}}}
 	// write: {"jsonrpc":"2.0","id":1,"method":"server/discover","params":{"_meta":{"io.modelcontextprotocol/clientCapabilities":{"roots":{"listChanged":true}},"io.modelcontextprotocol/clientInfo":{"name":"client","version":"v0.0.1"},"io.modelcontextprotocol/protocolVersion":"2026-07-28"}}}
 }