docs: add catalog REST headers section

[karangoel16]

Summary

• Add the missing HTTP Headers section to the Catalog REST binding
• Render request and response headers from the existing search_catalog OpenAPI operation
• Document the UCP-Agent catalog REST request header requirement

Validation

• python3 scripts/validate_examples.py --schema-base source/schemas/ --file docs/specification/catalog/rest.md
• Verified header_fields('search_catalog', 'rest.openapi.json') renders response headers including Content-Digest

[damaz91]

Thanks! This LGTM