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

jsonrpc/jsonrpc.go

@@ -17,6 +17,8 @@ type (
 	Request = jsonrpc2.Request
 	// Response is a JSON-RPC response.
 	Response = jsonrpc2.Response
+	// Error is a structured error in a JSON-RPC response.
+	Error = jsonrpc2.WireError
 )
 
 // MakeID coerces the given Go value to an ID. The value should be the
@@ -37,3 +39,18 @@ func EncodeMessage(msg Message) ([]byte, error) {
 func DecodeMessage(data []byte) (Message, error) {
 	return jsonrpc2.DecodeMessage(data)
 }
+
+// Standard JSON-RPC 2.0 error codes.
+// See https://www.jsonrpc.org/specification#error_object
+const (
+	// CodeParseError indicates invalid JSON was received by the server.
+	CodeParseError = -32700
+	// CodeInvalidRequest indicates the JSON sent is not a valid Request object.
+	CodeInvalidRequest = -32600
+	// CodeMethodNotFound indicates the method does not exist or is not available.
+	CodeMethodNotFound = -32601
+	// CodeInvalidParams indicates invalid method parameter(s).
+	CodeInvalidParams = -32602
+	// CodeInternalError indicates an internal JSON-RPC error.
+	CodeInternalError = -32603
+)