Support resource links in tool result content

[ptitjes]

Is your feature request related to a problem? Please describe.
Protocol 2025-06-18 allows a new type resource_link of tool result content: https://modelcontextprotocol.io/specification/2025-06-18/server/tools#resource-links

It is only provided for tool results and not for prompt results, so supporting this might need to modify the PromptMessageContent sealed hierarchy.

Describe the solution you'd like

• Make a new ToolResultContent sealed hierarchy that share sub-classes with the PromptMessageContent sealed hierarchy.
• Add a new ResourceLinkContent subclass to that new sealed hiearchy.

Describe alternatives you've considered
N/A

Additional context

• Protocol 2025-06-18 > Key Changes
• Tools > Data Types > Tool Result > Resource Links

[ptitjes]

I can PR for this.