refactor: move HostedMCPServer from core package to examples folder

- Move hosted_server.py to examples/python/custom_tools/
- Update import in mcp_server_latency_example.py to use local file
- Remove HostedMCPServer from core package exports

This keeps HostedMCPServer as an example implementation rather than
a core package component, as requested.

Co-authored-by: Mervin Praison <[email protected]>

Author: claude[bot]