docs(specification): add error response declarations to Shopping service definitions

[prasad-stripe]

Summary

The REST OpenAPI and MCP OpenRPC Shopping service definitions currently
declare only success responses (200/201). This PR adds standard error
response declarations to every endpoint:

• 400 on POST/PUT operations with a request body
• 401 on all operations
• 404 on operations addressing a resource by ID
• 409 on operations requiring Idempotency-Key

All error responses reference the existing error_response.json schema.
For MCP, equivalent JSON-RPC error objects are defined in components/errors.

Motivation

• Code generators use response declarations to produce typed error handling
• Implementers need normative guidance on expected error conditions
• The error_response.json schema already exists but was not surfaced at the service layer

Changes

• source/services/shopping/rest.openapi.json: add 4xx responses (pure additions)
• source/services/shopping/mcp.openrpc.json: add components/errors + per-method error refs