fix(go/plugins/mcp): Retrieve MCP server output schema

[pfernandom]

Currently, tool definitions retrieved from an MCP server using GentkitMCPClient method GetActiveTools contain only an empty map at tool.Definition().OutputSchema, even if the server explicitly declares an output schema for the given tool.

e.g. For the following MCP response:

{
  "tools": [
    {
      "name": "embed",
      "description": "Embed a text into a vector",
      "inputSchema": {
        "properties": { "text": { "type": "string" } },
        "required": ["text"],
        "type": "object"
      },
      "outputSchema": {
        "properties": {
          "result": { "items": { "type": "number" }, "type": "array" }
        },
        "required": ["result"],
        "type": "object",
        "x-fastmcp-wrap-result": true
      },
      "_meta": { "_fastmcp": { "tags": [] } }
    }
  ]
}

The following data is included in the tool definition:

This is in part because the current version for github.com/mark3labs/mcp-go doesn't include the OutputSchema attribute in the struct used to deserialize the message:

This PR introduces the following changes:

• Update github.com/mark3labs/mcp-go to the latest version, which does include OutputSchema in the Tool struct.
• Update the genkit MCP functions to actually retrieve the schema and use it during tool retrieval.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)

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