internal/mcp: add a test for complete mapping of CallToolParams fields

Add a test to verify that the toWireParams helper maps all fields of
CallToolParams.

Change-Id: I81c28791734d60c793c76809c6b93d27e78c3c36
Reviewed-on: https://go-review.googlesource.com/c/tools/+/677975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/client_test.go

@@ -7,6 +7,7 @@ package mcp
 import (
 	"context"
 	"fmt"
+	"reflect"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -199,3 +200,28 @@ func TestClientPaginateVariousPageSizes(t *testing.T) {
 		})
 	}
 }
+
+func TestToWireParams(t *testing.T) {
+	// This test verifies that toWireParams maps all fields.
+	// The Meta and Arguments fields are not comparable, so can't be checked by
+	// this simple test. However, this test will fail if new fields are added,
+	// and not handled by toWireParams.
+	params := &CallToolParams[struct{}]{
+		Name: "tool",
+	}
+	wireParams, err := toWireParams(params)
+	if err != nil {
+		t.Fatal(err)
+	}
+	v := reflect.ValueOf(wireParams).Elem()
+	for i := range v.Type().NumField() {
+		f := v.Type().Field(i)
+		if f.Name == "Meta" || f.Name == "Arguments" {
+			continue // not comparable
+		}
+		fv := v.Field(i)
+		if fv.Interface() == reflect.Zero(f.Type).Interface() {
+			t.Fatalf("toWireParams: unmapped field %q", f.Name)
+		}
+	}
+}