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/streamable_test.go

@@ -962,7 +962,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					method:         "POST",
 					messages:       []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Name: "tool"})},
 					wantStatusCode: http.StatusOK,
-					wantMessages: []jsonrpc.Message{resp(2, nil, &jsonrpc2.WireError{
+					wantMessages: []jsonrpc.Message{resp(2, nil, &jsonrpc.Error{
 						Message: `method "tools/call" is invalid during session initialization`,
 					})},
 				},