Ping request from non-Dart client leads to error response.

[ksokolovskyi]

Steps to reproduce

1. Use dart_mcp: 0.3.2
2. Build any MCP server from the dart_mcp/example directory (e.g. `dart_mcp/example/tools_server.dart)
3. Connect the built server to any non-Dart MCP Client
4. Send a ping request, and you will receive the error

Expected results

An empty response is getting received.

Actual results

Error response is getting received.

{
   "error": "MCP error -32000: type 'CastMap<String, dynamic, String, Object?>' is not a subtype of type 'Null' in   type cast"
}

Screenshots or Video

ping.mp4

[jakemac53]

This is a bit interesting because the textual specification and the JSON schema specification disagree a bit here:

https://modelcontextprotocol.io/specification/2025-06-18/basic/utilities/ping#message-format says explicitly "with no parameters", but the schema has a params object defined https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-06-18/schema.json#L1464.

I do agree we should be handling the params in this case, and thanks for the pr to go with the issue!