Allow for templated variable cascading scenarios across packages and remotes

Follow on to https://github.com/modelcontextprotocol/registry/pull/570

See https://github.com/modelcontextprotocol/registry/pull/570#issuecomment-3367417636 for the starting point

My rough understanding of what we want to accomplish here is to, at minimum:

1. Collect nested variables in the validators so that parametrized schemas like this are allowed:

{
  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-09-16/server.schema.json",
  "name": "io.github.SamYuan1990/i18n-agent-action",
  "description": "An i18n github action for language translate",
  "version": "mcp",
  "packages": [
    {
      "registryType": "oci",
      "registryBaseUrl": "https://ghcr.io",
      "identifier": "SamYuan1990/i18n-agent-action",
      "version": "mcp",
      "runtimeHint": "docker",
      "transport": {
        "type": "sse",
        "url": "https://example.com:{host_port}/sse"
      },
      "runtimeArguments": [
        {
          "description": "Port mapping from host to container",
          "value": "{host_port}:8080",
          "type": "named",
          "name": "-p",
          "variables": {
            "host_port": {
              "description": "The host (local) port",
              "isRequired": true,
              "format": "number",
              "default": "8080"
            }
          }
        }
      ]
    }
  ]
}

2. Properly document the scenarios where these template variables cascade (and what to do in case of conflicts / which layers have precedence).

(2) Might potentially deserve an extension to syntax to do things like {-p.host_port} or {p.host_port} in order to disambiguate more explicitly. But my inclination is to not add the extra syntax (just do blind {host_port} with some precedence order of where to look), as this won't be a commonly used feature and some reasonable precedence cascade would be sufficiently intuitive that users don't have to learn the syntax and can just intuit how variables would cascade.

Start with pull request #570 and its linked comment, then trace the validators that process nested package and runtime variables. Define and document precedence and conflict behavior for cascading template variables, and verify the parametrized schema example. Whether to add explicit variable syntax remains an open design question.