jsonschema: implement JSON Pointers for schemas

Implement enough of the JSON Pointer spec to suffice for JSON Schema.

For details, see the comment at the top of json_pointer.go.

Change-Id: Ic297f1a1a08903b4b3c6666fe57ac1f45c96c0f7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670535
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/json_pointer_test.go

@@ -0,0 +1,78 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package jsonschema
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDereferenceJSONPointer(t *testing.T) {
+	s := &Schema{
+		AllOf: []*Schema{{}, {}},
+		Defs: map[string]*Schema{
+			"":  {Properties: map[string]*Schema{"": {}}},
+			"A": {},
+			"B": {
+				Defs: map[string]*Schema{
+					"X": {},
+					"Y": {},
+				},
+			},
+			"/~": {},
+			"~1": {},
+		},
+	}
+
+	for _, tt := range []struct {
+		ptr  string
+		want any
+	}{
+		{"", s},
+		{"/$defs/A", s.Defs["A"]},
+		{"/$defs/B", s.Defs["B"]},
+		{"/$defs/B/$defs/X", s.Defs["B"].Defs["X"]},
+		{"/$defs//properties/", s.Defs[""].Properties[""]},
+		{"/allOf/1", s.AllOf[1]},
+		{"/$defs/~1~0", s.Defs["/~"]},
+		{"/$defs/~01", s.Defs["~1"]},
+	} {
+		got, err := dereferenceJSONPointer(s, tt.ptr)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got != tt.want {
+			t.Errorf("%s:\ngot  %+v\nwant %+v", tt.ptr, got, tt.want)
+		}
+	}
+}
+
+func TestDerefernceJSONPointerErrors(t *testing.T) {
+	s := &Schema{
+		Type:     "t",
+		Items:    &Schema{},
+		Required: []string{"a"},
+	}
+	for _, tt := range []struct {
+		ptr  string
+		want string // error must contain this string
+	}{
+		{"x", "does not begin"}, // parse error: no initial '/'
+		{"/minItems", "does not refer to a schema"},
+		{"/minItems/x", "navigated to nil"},
+		{"/required/-", "not supported"},
+		{"/required/01", "leading zeroes"},
+		{"/required/x", "invalid int"},
+		{"/required/1", "out of bounds"},
+		{"/properties/x", "no key"},
+	} {
+		_, err := dereferenceJSONPointer(s, tt.ptr)
+		if err == nil {
+			t.Errorf("%q: succeeded, want failure", tt.ptr)
+		} else if !strings.Contains(err.Error(), tt.want) {
+			t.Errorf("%q: error is %q, which does not contain %q", tt.ptr, err, tt.want)
+		}
+	}
+}