docs: add creative agent setup verification

[sangilish]

Summary

• add a deployment-checklist assertion for non-empty creative.supported_formats
• document a vendor-neutral MCP get_adcp_capabilities connectivity probe
• show the canonical format declaration shape buyers should expect

Why

Creative-agent implementers and buyers have no runnable setup check in the deployment guide. The new probe verifies endpoint reachability and the canonical creative-format discovery surface without depending on a specific vendor or the deprecated list_creative_formats discovery path.

Closes #5877.

Validation

• node tests/snippet-validation.test.cjs --file docs/creative/implementing-creative-agents.mdx
• node tests/docs-nav-validation.test.cjs
• npm run check:seo (passed with existing repository warnings)
• git diff --check

No changeset: documentation-only, non-normative addition.

[aao-secretariat]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Initialize MCP sessions before probing tools

For MCP endpoints that implement the standard lifecycle or return an Mcp-Session-Id, this raw tools/call as the first request will be rejected or run before capabilities are negotiated; the MCP lifecycle spec says initialization is the first interaction and Streamable HTTP clients must carry the returned session ID on subsequent requests (see https://modelcontextprotocol.io/specification/2025-03-26/basic/lifecycle#initialization and https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#session-management). Since this is presented as the runnable deployment check for remote MCP creative agents, compliant stateful servers can appear broken. Add the initialize/initialized handshake and session header steps, or point readers to an MCP client/CLI that performs them.

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