internal/mcp/jsonschema: check for structure separately

Redo iteration. It was too subtle.

Write a separate recursive check for whether the schemas form a tree and
have any invalid nils.

While doing that, assign paths to schemas for better error messages,
even as we check structure. This also lets us detect re-occurring
schemas without a separate map.

Everything else, including the iterators, can now assume valid structure
and populated paths.

Change-Id: I90ec93d79d1d95c92258ad7afc94ef068fdf123f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>

jsonschema/schema_test.go

@@ -9,8 +9,6 @@ import (
 	"fmt"
 	"math"
 	"regexp"
-	"slices"
-	"strings"
 	"testing"
 )
 
@@ -111,37 +109,6 @@ func TestUnmarshalErrors(t *testing.T) {
 	}
 }
 
-func TestEvery(t *testing.T) {
-	// Schema.every should visit all descendants of a schema, not just the immediate ones.
-	s := &Schema{
-		Type:        "string",
-		PrefixItems: []*Schema{{Type: "int"}, {Items: &Schema{Type: "null"}}},
-		Contains: &Schema{Properties: map[string]*Schema{
-			"~1": {Type: "boolean"},
-			"p":  {},
-		}},
-	}
-
-	type item struct {
-		s *Schema
-		p string
-	}
-	want := []item{
-		{s, ""},
-		{s.Contains, "contains"},
-		{s.Contains.Properties["p"], "contains/properties/p"},
-		{s.Contains.Properties["~1"], "contains/properties/~01"},
-		{s.PrefixItems[0], "prefixItems/0"},
-		{s.PrefixItems[1], "prefixItems/1"},
-		{s.PrefixItems[1].Items, "prefixItems/1/items"},
-	}
-	var got []item
-	s.every(func(s *Schema, p []string) bool { got = append(got, item{s, strings.Join(p, "/")}); return true }, nil)
-	if !slices.Equal(got, want) {
-		t.Errorf("\n got  %v\nwant %v", got, want)
-	}
-}
-
 func mustUnmarshal(t *testing.T, data []byte, ptr any) {
 	t.Helper()
 	if err := json.Unmarshal(data, ptr); err != nil {