Filter MCPServer capabilities by registered primitives

[MukundaKatta]

Summary

• stop MCPServer from advertising prompt, resource, and tool capabilities when nothing has been registered for that primitive
• keep the low-level Server behavior unchanged by making the filtering opt-in
• add coverage for empty, tool-only, prompt-only, and resource-template-only MCPServer initialization

Testing

• python3 -m py_compile src/mcp/server/lowlevel/server.py src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
• pytest tests/server/mcpserver/test_server.py -k capability (not run locally: pytest is not installed in this environment)

[rendina-io]

Advertising capabilities that have no registered handlers is a sneaky source of compatibility issues — clients that enumerate capabilities and then try to call an unregistered primitive get confusing 'method not found' errors instead of a clean capability negotiation failure. Filtering at the capability advertisement level is the right place to fix this. Does the change affect the list_changed notification behavior, or is that still gated on the same registered-primitive check?