internal/mcp/jsonschema: clearer validations errors

Validation errors now more clearly indicate the path from the root
schema.

- Enhance schema iteration to yield the path, and generalize it by using
  reflection.

- Store each schema's path inside the schema.

- Remove the path arg from validate: we don't need it anymore.

- Improve the test for the iterators and add one for validation
  errors.

Change-Id: Ief55133ef4ae2eac0736208a3b9f78ed69933b31
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676375
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>

jsonschema/validate_test.go

@@ -78,6 +78,21 @@ func TestValidate(t *testing.T) {
 	}
 }
 
+func TestValidateErrors(t *testing.T) {
+	schema := &Schema{
+		PrefixItems: []*Schema{{Contains: &Schema{Type: "integer"}}},
+	}
+	rs, err := schema.Resolve("", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = rs.Validate([]any{[]any{"1"}})
+	want := "prefixItems/0"
+	if err == nil || !strings.Contains(err.Error(), want) {
+		t.Errorf("error:\n%s\ndoes not contain %q", err, want)
+	}
+}
+
 func TestStructInstance(t *testing.T) {
 	instance := struct {
 		I int