Add Claude Desktop web search sample (AgentCore Gateway)

### What
Add a new sample under
01-features/03-connect-your-agent-to-anything/03-web-search/04-claude-desktop
showing how to connect Claude Desktop to AgentCore Web Search through an
AgentCore Gateway (MCP endpoint), with Cognito/IAM Identity Center federation.

### Why
Complements the existing web-search samples (raw MCP, Strands, LangChain) with a
no-code MCP client (Claude Desktop), and documents the security/governance
benefits of going through the Gateway (WAF, Guardrails, Cedar policy, no data egress).

### Deliverables
- provision.py + provision_web_search_gateway.ipynb
- README with setup, usage, and diagrams

Start by reviewing the existing raw MCP, Strands, and LangChain web-search samples, then create the new 04-claude-desktop sample under 01-features/03-connect-your-agent-to-anything/03-web-search/. The deliverables are provision.py, provision_web_search_gateway.ipynb, and a README covering setup, usage, security governance, and diagrams; the sample is done when it demonstrates Claude Desktop connecting through the AgentCore Gateway with Cognito/IAM Identity Center federation.

Written by the indexing model from the issue text.