Fix default handler return values to comply with MCP spec

The no-op handlers for `completion/complete`, `resources/subscribe`,
and `resources/unsubscribe` returned `nil`.
When a client accesses the expected response structure (e.g., `result[:completion][:values]`),
a `nil` result raises `NoMethodError`.

The MCP spec requires specific response structures even for no-op handlers.
Returning spec-compliant defaults ensures that clients receive valid responses without
requiring servers to explicitly override each handler.

`completion/complete` now returns `{ completion: { values: [], hasMore: false } }`.
ref: https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/completion

`resources/subscribe` and `resources/unsubscribe` now return `{}` (empty result).
ref: https://modelcontextprotocol.io/specification/2025-11-25/server/resources#subscriptions

lib/mcp/server.rb

@@ -99,9 +99,9 @@ module MCP
         Methods::LOGGING_SET_LEVEL => method(:configure_logging_level),
 
         # No op handlers for currently unsupported methods
-        Methods::RESOURCES_SUBSCRIBE => ->(_) {},
-        Methods::RESOURCES_UNSUBSCRIBE => ->(_) {},
-        Methods::COMPLETION_COMPLETE => ->(_) {},
+        Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
+        Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
+        Methods::COMPLETION_COMPLETE => ->(_) { { completion: { values: [], hasMore: false } } },
         Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport