[Documentation]: Add Unix socket and SSE transport types to MCP Integration Guide

[Ttian18]

Summary

The MCP client supports four transport types (stdio, http, unix, sse), but the MCP Integration Guide only documents STDIO and HTTP. Unix and SSE were added in PR #6531 but docs weren't updated.

Scope

1. Transport Types section — Add ### Unix Socket Transport and ### SSE Transport subsections after ### HTTP Transport, with configuration details and usage examples
2. Overview line (line 9) — Update "via STDIO or HTTP transport" to include all four
3. Best Practices section — Add guidance on when to choose Unix/SSE over HTTP/STDIO

Acceptance Criteria

• Unix socket transport is documented with config fields (url, socket_path) and example
• SSE transport is documented with config fields (url, headers) and example
• Overview and Best Practices sections updated to reflect all four transports

Relevant Files

• core/MCP_INTEGRATION_GUIDE.md
• core/framework/runner/mcp_client.py (reference implementation, lines 275–357)
• PR feat: add unix and sse MCP transports #6531

[Ttian18]

Hi, I'd like to work on this. Please assign it to me.