What does this PR do?

yamlSchemaService could keep expanding references forever and crash with heap/stack OOM when resolving schema with cyclic references.

Original implementation introduces a bug when $ref + siblings are rewritten to allOf and creates new nodes. This can cause dynamic ref paths to revisit the same resource repeatedly.

Fixed by:

• Added sibling cycle tracking in traversal state in WalkItem.siblingRefCycleKeys
• Stop rewriting when the same sibling-ref target is seen again
• Added a helper to short-circuit when a target resource is already in resolutionStack
• Changed local merge cloning to only clone for local $dynamicRef with dynamic scope

What issues does this PR fix or reference?

Fixes #1190

Is it tested? How?

Automated test and manually with this example:

# yaml-language-server: $schema=https://raw.githubusercontent.com/OAI/spec.openapis.org/refs/heads/main/oas/3.1/schema/2025-11-23
openapi: "3.1.0"
info:
  version: 1.0.0
  title: Minimal ping API server
paths:
  /ping:
    get:
      responses:
        "200":
          description: pet response
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Pong"
components:
  schemas:
    # base types
    Pong:
      type: object
      required:
        - ping
      properties:
        ping:
          type: string
          example: pong

When triggering auto-completion, expected to see 'no suggestion' instead of no response (keep loading).