feat: add Xquik MCP example

[kriptoburak]

Summary

• add a remote Xquik MCP example using Agentflow's existing ToolNode boundary
• update 3 MCP examples to use the current tool_node constructor keyword
• add a static regression test for the corrected examples

The integration is opt-in, reads the API key from the environment, and does not change framework defaults.

Validation

• focused pytest: 1 passed
• Python compile check for all changed examples
• MCP client and ToolNode construction smoke check
• README link check
• confidentiality and diff hygiene checks

Duplicate Check

Checked the default branch, all-state pull requests and issues, and discussions for Xquik names, domains, and aliases. No existing placement or active duplicate was found.