docs(access-requests): describe resource_reference as alternative to resource_id in schema

[juliahooper]

Summary

• Adds descriptions to resource_id and resource_reference fields on AccessRequestItemRequest
• Makes it clear that resource_reference with {vendor, name, version} can be used instead of a toolkit ID for toolkit:bind requests
• Agents previously only discovered this flow after hitting the broker's no_toolkit_binding error

Test plan

• ruff check + mypy clean
• All unit + arch tests pass (1929 passed)
• OpenAPI spec and UI client regenerated