fix(schema): remove null nextCursor from MCP

[rozzilla]

The handleToolsList, handleResourcesList, and handlePromptsList handlers
explicitly set nextCursor: undefined in their response objects. When Fastify
serializes these through fast-json-stringify, undefined values are converted
to null, producing "nextCursor": null in the JSON output.

This violates the MCP specification JSON Schema,
which defines nextCursor as:

"nextCursor": {
  "description": "An opaque token representing the pagination position after the last returned result.",
  "type": "string"
}

[mcollina]

lgtm

[mcollina]

When Fastify
serializes these through fast-json-stringify, undefined values are converted
to null, producing "nextCursor": null in the JSON output.

I'm relatively certain this is not the case, at least alone.

[rozzilla]

When Fastify
serializes these through fast-json-stringify, undefined values are converted
to null, producing "nextCursor": null in the JSON output.

I'm relatively certain this is not the case, at least alone.

this is the error I received from the tool when I tried to connect to @platformatic/mcp:

{
    "error": {
        "type": "invalid_request_error",
        "message": "Error fetching remote server metadata: [\n  {\n    \"expected\": \"string\",\n    \"code\": \"invalid_type\",\n    \"path\": [\n      \"nextCursor\"\n    ],\n    \"message\": \"Invalid input: expected string, received null\"\n  }\n]"
    }
}

I suspect, but I can't garantee 100%, though this is coming exactly from fast-json-stringify

[mcollina]

The tests you added pass without the changes in src.