feat: multi-round-trip request implementation (SEP-2322) (#950)

### Context

Design (`design/mrtr.md`) and implementation proposal for Multi
Round-Trip Requests (MRTR) per
[SEP-2322](de6d76fba3/seps/2322-MRTR.md).

### Changes

- Introduce `InputRequest` and `InputResponse` sealed interfaces with
corresponding `InputRequestMap` and `InputResponseMap` types for custom
map JSON codec implementation.
- Extend `CallToolParams`, `GetPromptParams`, and `ReadResourceParams`
with `InputResponses`, `RequestState` and an unexported `resultType`
("complete" or "input_required") for retry round-trips. The type is set
by the SDK based on input requests presence.
- Add client-side MRTR middleware enabled by default that automatically
fulfills input requests and retries.
- Added MRTROptions on ClientOptions to allow disabling the middleware
and configuring the number of retries.
- Add server-side MRTR middleware for backward compatibility:
transparently bridges input requests to direct
Elicit/CreateMessage/ListRoots calls for older clients

mcp/streamable_test.go

@@ -769,6 +769,12 @@ func req(id int64, method string, params any) *jsonrpc.Request {
 	return r
 }
 
+func completeCallToolResult() *CallToolResult {
+	r := &CallToolResult{Content: []Content{}}
+	r.resultType = resultTypeComplete
+	return r
+}
+
 func resp(id int64, result any, err error) *jsonrpc.Response {
 	return &jsonrpc.Response{
 		ID:     jsonrpc2.Int64ID(id),
@@ -1947,7 +1953,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 			},
 			messages:       []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Meta: testMeta, Name: "my-tool"})},
 			wantStatusCode: http.StatusOK,
-			wantMessages:   []jsonrpc.Message{resp(2, &CallToolResult{Content: []Content{}}, nil)},
+			wantMessages:   []jsonrpc.Message{resp(2, completeCallToolResult(), nil)},
 		},
 		{
 			method: "POST",
@@ -1991,7 +1997,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 			},
 			messages:       []jsonrpc.Message{req(6, "tools/call", &CallToolParams{Meta: testMeta, Name: "my-tool"})},
 			wantStatusCode: http.StatusOK,
-			wantMessages:   []jsonrpc.Message{resp(6, &CallToolResult{Content: []Content{}}, nil)},
+			wantMessages:   []jsonrpc.Message{resp(6, completeCallToolResult(), nil)},
 		},
 		{
 			method: "POST",
@@ -2007,7 +2013,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 				Arguments: map[string]any{"region": "us-west1", "query": "SELECT 1"},
 			})},
 			wantStatusCode: http.StatusOK,
-			wantMessages:   []jsonrpc.Message{resp(7, &CallToolResult{Content: []Content{}}, nil)},
+			wantMessages:   []jsonrpc.Message{resp(7, completeCallToolResult(), nil)},
 		},
 		{
 			method: "POST",