fix mcp right name in docs

[anaslimem]

Summary

Fixes incorrect field name mcp → mcpCapabilities in the initialization documentation example.

Changes

• docs/protocol/initialization.mdx: Changed "mcp": to "mcpCapabilities": in the initialize response example (line 71)

Problem

The JSON example in the initialization documentation showed an incorrect field name:

"agentCapabilities": {
  "mcp": {        // Incorrect
    "http": true,
    "sse": true
  }
}

This was inconsistent with:

• The JSON schema (schema/schema.json defines mcpCapabilities)
• The Rust implementation (src/agent.rs defines mcp_capabilities field)
• All other documentation examples (e.g., docs/protocol/session-setup.mdx)