test: make persisted agent registry smoke order-insensitive

[chubes4]

Problem

On current origin/main (b1adfe2cf, after #3420), php tests/agents-api-persisted-agent-registry-smoke.php fails two ordering-only assertions.

Expected:
wordpress-com-wiki, woocommerce-wiki, roadie, coordinator

Actual:
wordpress-com-wiki, coordinator, woocommerce-wiki, roadie

All individual projection contracts pass, including canonical wp_get_agents() visibility, wp_get_agent() lookup, labels, descriptions, owner resolution, config, metadata, subagents, duplicate preservation, and no duplicate notices.

Scope

• Confirm whether persisted repository ordering is contractual.
• If ordering is not contractual, compare the projected slug set rather than insertion order.
• If ordering is contractual, make the projector explicitly establish it and document the contract.

Verification

• Run php tests/agents-api-persisted-agent-registry-smoke.php.
• Run the owning managed WordPress suite.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode identified and reproduced this existing baseline test defect while verifying #3421 under Chris Huber’s direction.