Fix MCP response format docs to use flat structure

[nastassiafulconis]

Summary

• Update MCP response format documentation to show flat structure (task-specific fields at top level) instead of nested data wrapper
• This aligns documentation with what @adcp/client actually expects

Context

Swivel was implementing their MCP salesagent following our docs, but their implementation didn't match what @adcp/client expects. The docs showed:

{
  "status": "completed",
  "data": {
    "products": [...]
  }
}

But @adcp/client expects:

{
  "status": "completed",
  "products": [...]
}

Changes

Files updated:

• docs/protocols/mcp-guide.mdx - Updated response format section and examples
• docs/protocols/core-concepts.mdx - Updated response structure and code examples
• docs/protocols/getting-started.mdx - Updated MCP quick comparison example
• docs/protocols/protocol-comparison.mdx - Updated all MCP response examples
• docs/protocols/a2a-guide.mdx - Fixed incorrect response.data usage in A2A context

Key changes:

• response.data.products → response.products
• response.data.media_buy_id → response.media_buy_id
• response.data?.errors → response.errors

Note: A2A examples correctly remain with {"kind": "data", "data": {...}} inside artifact parts, which is proper A2A protocol structure.

Test plan

• All schema validation tests pass
• All example validation tests pass
• Build succeeds
• Typecheck passes

🤖 Generated with Claude Code

[github-actions]

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