internal/mcp: implement resources

Implementation of resources.

This follows the design sketched in design/design.md, except for
Server.FileResourceHandler, which will be in a forthcoming CL.

As the spec recommends, the server verifies an incoming URI from the
client against its list of resources, to make sure the URI came from one
of them.

Some things that aren't in the design doc:
- I provide an exported function that creates a "resource not found"
  jsonrpc2.WireEror error for server authors to return.
  A wrinkle: the MCP spec defines the error code for
  "resource not found" as -32002 [1]. That code is reserved for use
  by implementations of JSON-RPC, not applications. That matters to us
  because our JSON-RPC implementation already uses that code for the
  "server closing" error. I filed a bug against MCP about it [2] and
  chose a different code for the "resource not found" error.

- As a slight convenience, I fill in some fields in the
  ReadResourceResult if the handler doesn't set them.

[1] https://modelcontextprotocol.io/specification/2025-03-26/server/resources#error-handling
[2] https://github.com/modelcontextprotocol/modelcontextprotocol/issues/509

Change-Id: I06153da982ea00748a3691ded923448a880cea2a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671363
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

mcp/client.go

@@ -221,6 +221,16 @@ func (c *Client) CallTool(ctx context.Context, name string, args map[string]any)
 	return standardCall[protocol.CallToolResult](ctx, c.conn, "tools/call", params)
 }
 
+// ListResources lists the resources that are currently available on the server.
+func (c *Client) ListResources(ctx context.Context, params *protocol.ListResourcesParams) (*protocol.ListResourcesResult, error) {
+	return standardCall[protocol.ListResourcesResult](ctx, c.conn, "resources/list", params)
+}
+
+// ReadResource ask the server to read a resource and return its contents.
+func (c *Client) ReadResource(ctx context.Context, params *protocol.ReadResourceParams) (*protocol.ReadResourceResult, error) {
+	return standardCall[protocol.ReadResourceResult](ctx, c.conn, "resources/read", params)
+}
+
 func standardCall[TRes, TParams any](ctx context.Context, conn *jsonrpc2.Connection, method string, params TParams) (*TRes, error) {
 	var result TRes
 	if err := call(ctx, conn, method, params, &result); err != nil {