$id fragment lookup silently returns the parent schema when resolution fails #676

On libopenapi v0.41.8, a reference resolved through a registered $id can return the enclosing schema when the requested fragment does not resolve. This affects both the absolute URI and its absolute-path lookup form.

With the schema below, #/properties/A+B%20C should select the integer property. #/properties/missing and #/properties/%zz should not resolve. All three currently return the enclosing object schema instead.

This silently changes the referenced contract and can hide a misspelled or malformed reference. The documented FindComponent/ResolveRefViaSchemaId behavior is to return nil when the target cannot be resolved.

Reproduction (Go 1.27.3, github.com/pb33f/libopenapi v0.41.8, github.com/pb33f/go-yaml v0.1.1):

Save as main.go in an empty directory, then run:

go mod init example.com/schema-id-fragment
go get github.com/pb33f/libopenapi@v0.41.8 github.com/pb33f/go-yaml@v0.1.1
go run .

package main

import (
	"context"
	"fmt"
	"github.com/pb33f/go-yaml"
	"github.com/pb33f/libopenapi/index"
)

func main() {
	const source = `openapi: 3.1.1
info: {title: Example, version: '1'}
paths: {}
components:
  schemas:
    Holder:
      $id: https://example.com/holder
      type: object
      properties:
        'A+B C': {type: integer}
`
	var root yaml.Node
	if err := yaml.Unmarshal([]byte(source), &root); err != nil {
		panic(err)
	}
	idx := index.NewSpecIndex(&root)
	for _, base := range []string{"https://example.com/holder", "/holder"} {
		for _, fragment := range []string{"#/properties/A+B%20C", "#/properties/missing", "#/properties/%zz"} {
			ref := idx.FindComponent(context.Background(), base+fragment)
			kind := "<nil>"
			if ref != nil && ref.Node != nil {
				for i := 0; i+1 < len(ref.Node.Content); i += 2 {
					if ref.Node.Content[i].Value == "type" {
						kind = ref.Node.Content[i+1].Value
					}
				}
			}
			fmt.Printf("%s%s -> %s\n", base, fragment, kind)
		}
	}
}

Actual: all six calls print object. Expected: integer for the encoded property name, <nil> for the missing member and malformed escape, for both URI forms.

The $id resolver retains entry.SchemaNode when fragment navigation fails and does not URI-decode the fragment before navigation. The absolute-path lookup repeats the parent fallback. Related pointer-token corrections are in #675, but these $id resolution paths bypass that lookup boundary and need their own correction.