Add ResourceLink content type support to vmcp bridge

[jhrozek]

Summary

The vmcp bridge layer (pkg/vmcp/conversion/content.go and pkg/vmcp/server/adapter/handler_factory.go) handles bidirectional conversion between MCP SDK types and the internal vmcp.Content type. It currently supports TextContent, ImageContent, AudioContent, and EmbeddedResource, but does not handle ResourceLink — a distinct content type introduced in the MCP 2025-06-18 spec.

Any backend MCP server returning resource_link content in tool results will have it silently converted to empty text, losing the URI, name, description, and MIME type.

Details

ResourceLink (type discriminant: "resource_link") is a pointer/reference to a resource, as opposed to EmbeddedResource which carries the resource contents inline. It's defined in both the 2025-06-18 and 2025-11-25 MCP specs and is fully supported in mcp-go v0.44.0:

• Type: mcp.ResourceLink (mcp/types.go:1117)
• Constructor: mcp.NewResourceLink(uri, name, description, mimeType) (mcp/utils.go:252)
• Constant: mcp.ContentTypeLink = "resource_link" (mcp/consts.go:7)
• JSON unmarshal: handled in UnmarshalContent (mcp/types.go:1567)
• No AsResourceLink() helper — requires direct type assertion: content.(mcp.ResourceLink)

ResourceLink fields (per SDK)

type ResourceLink struct {
    Annotated
    Type        string `json:"type"`        // "resource_link"
    URI         string `json:"uri"`
    Name        string `json:"name"`
    Description string `json:"description"`
    MIMEType    string `json:"mimeType"`
}

What needs to change

1. Extend vmcp.Content (pkg/vmcp/types.go) — add Name and Description fields (or introduce a new type/approach)
2. MCP → vmcp direction (pkg/vmcp/conversion/content.go) — add a ResourceLink branch in ConvertMCPContent
3. vmcp → MCP direction (pkg/vmcp/server/adapter/handler_factory.go) — add a "resource_link" case in convertToMCPContent
4. Tests for both directions

Current behavior

// content.go — ResourceLink falls through to unknown
slog.Debug("Encountered unknown MCP content type", ...)
return vmcp.Content{Type: "unknown"}

// handler_factory.go — "unknown" becomes empty text
slog.Warn("converting unknown content type to empty text - this may cause data loss", ...)
return mcp.NewTextContent("")

Found during review of #3953.

[Sanskarzz]

I'd like to work on this

[jerm-dro]

@Sanskarzz assigned!