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/client_test.go

@@ -222,6 +222,61 @@ func TestClientCapabilities(t *testing.T) {
 				Sampling: &SamplingCapabilities{},
 			},
 		},
+		{
+			name:            "With form elicitation",
+			configureClient: func(s *Client) {},
+			clientOpts: ClientOptions{
+				ElicitationModes: []string{"form"},
+				ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
+					return nil, nil
+				},
+			},
+			wantCapabilities: &ClientCapabilities{
+				Roots: struct {
+					ListChanged bool "json:\"listChanged,omitempty\""
+				}{ListChanged: true},
+				Elicitation: &ElicitationCapabilities{
+					Form: &FormElicitationCapabilities{},
+				},
+			},
+		},
+		{
+			name:            "With URL elicitation",
+			configureClient: func(s *Client) {},
+			clientOpts: ClientOptions{
+				ElicitationModes: []string{"url"},
+				ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
+					return nil, nil
+				},
+			},
+			wantCapabilities: &ClientCapabilities{
+				Roots: struct {
+					ListChanged bool "json:\"listChanged,omitempty\""
+				}{ListChanged: true},
+				Elicitation: &ElicitationCapabilities{
+					URL: &URLElicitationCapabilities{},
+				},
+			},
+		},
+		{
+			name:            "With both form and URL elicitation",
+			configureClient: func(s *Client) {},
+			clientOpts: ClientOptions{
+				ElicitationModes: []string{"form", "url"},
+				ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
+					return nil, nil
+				},
+			},
+			wantCapabilities: &ClientCapabilities{
+				Roots: struct {
+					ListChanged bool "json:\"listChanged,omitempty\""
+				}{ListChanged: true},
+				Elicitation: &ElicitationCapabilities{
+					Form: &FormElicitationCapabilities{},
+					URL:  &URLElicitationCapabilities{},
+				},
+			},
+		},
 	}
 
 	for _, tc := range testCases {