Provide an option to *not* generate content when structured content is returned

[jlowin]

Enhancement

Currently, tools that infer structured content also generate serialized content payloads. This is compliant with the MCP spec guidance of what SHOULD be done for backwards compatibility. See https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

However in some cases users might want to disable this behavior globally. (On a per tool basis it is already possible by returning ToolResults I believe)