all: expose JSON-RPC error codes, and support URLElicitationRequired errors (#655)

Expose the jsonrpc.Error type to allow access to underlying JSON-RPC
error codes. Also, expose common JSON-RPC error codes in the jsonrpc
package, and MCP error codes in the mcp package.

Also, implement support for the MCP URL elicitation required error
(-32042), enabling servers to request out-of-band user authorization and
clients to automatically handle these requests with retry logic.

Server-side:
- Add CodeURLElicitationRequired constant
- Add URLElicitationRequiredError() constructor.

Client-side:
- Add urlElicitationMiddleware to intercept URLElicitationRequired
  errors and retry calls.
- Update callElicitationCompleteHandler to signal waiting operations.
- Fix capability advertisement to support both form and URL modes.

Fixes #452
Fixes #623

mcp/resource.go

@@ -15,8 +15,8 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 	"github.com/modelcontextprotocol/go-sdk/internal/util"
+	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
 	"github.com/yosida95/uritemplate/v3"
 )
 
@@ -40,8 +40,8 @@ type ResourceHandler func(context.Context, *ReadResourceRequest) (*ReadResourceR
 // ResourceNotFoundError returns an error indicating that a resource being read could
 // not be found.
 func ResourceNotFoundError(uri string) error {
-	return &jsonrpc2.WireError{
-		Code:    codeResourceNotFound,
+	return &jsonrpc.Error{
+		Code:    CodeResourceNotFound,
 		Message: "Resource not found",
 		Data:    json.RawMessage(fmt.Sprintf(`{"uri":%q}`, uri)),
 	}