Let mcpcompat resource handlers return _meta

[jhrozek]

Part of stacklok/toolhive#6027.

Problem

mcpcompat server resource handlers cannot return _meta, so any backend
resource metadata is dropped before it reaches the wire — even though go-sdk
supports the field.

// server/server.go:70
type ResourceHandlerFunc func(ctx context.Context, request mcp.ReadResourceRequest) ([]mcp.ResourceContents, error)

The bridge to go-sdk then builds a result with no meta to populate:

// server/server.go:1024-1043, wrapResourceHandler
contents, err := h(ctx, mreq)
...
jsonConvert(mcp.ReadResourceResult{Contents: contents}, out)  // Meta always nil

Both ends already support it — only the handler signature is in the way:

• go-sdk: ResourceHandler func(ctx, *ReadResourceRequest) (*ReadResourceResult, error),
  and ReadResourceResult embeds Meta (mcp/resource.go:39, mcp/protocol.go:1595).
• mcpcompat: mcp.ReadResourceResult embeds Result, which carries Meta
  (mcp/resources.go:191).

The []ResourceContents shape came from mark3labs/mcp-go and was kept for
source compatibility during the migration.

Proposal

Add a result-returning variant alongside the existing handler — e.g. a
ResourceResultHandlerFunc returning (*mcp.ReadResourceResult, error) plus
AddResourceWithResult / an equivalent registration path, and the same for
resource templates (wrapResourceHandler is shared by
AddResourceTemplate via server.go:608).

wrapResourceHandler then forwards the handler's Meta into the go-sdk result
instead of constructing a meta-less one.

Additive on purpose. Existing ResourceHandlerFunc callers keep compiling.
This does not weaken the mcp-go source-compatibility contract: mcp-go has no
result-returning resource handler, so there is no upstream shape to diverge
from.

Consumers waiting on this

In ToolHive, both drop resource _meta for this reason today:

• pkg/vmcp/server/serve_handlers.go — coreResourceHandler /
  coreResourceTemplateHandler
• pkg/transport/bridge.go:226,241

Tracked downstream in stacklok/toolhive#6027.

Acceptance

• A resource handler can set _meta and it appears on the resources/read wire result.
• Existing ResourceHandlerFunc registrations are unchanged and still compile.
• Covered for both AddResource and AddResourceTemplate.

Generated with Claude Code