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/protocol_test.go

@@ -1151,6 +1151,71 @@ func TestToWithTools_Conversion(t *testing.T) {
 	}
 }
 
+func TestInputRequestMapJSON(t *testing.T) {
+	t.Run("nil is omitted from JSON", func(t *testing.T) {
+		result := CallToolResult{Content: []Content{&TextContent{Text: "ok"}}}
+		data, err := json.Marshal(&result)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var raw map[string]json.RawMessage
+		if err := json.Unmarshal(data, &raw); err != nil {
+			t.Fatal(err)
+		}
+		if _, ok := raw["inputRequests"]; ok {
+			t.Errorf("nil InputRequests should be omitted, got %s", raw["inputRequests"])
+		}
+	})
+
+	t.Run("non-nil empty round-trips", func(t *testing.T) {
+		result := CallToolResult{
+			Content:       []Content{&TextContent{Text: "ok"}},
+			InputRequests: InputRequestMap{},
+		}
+		data, err := json.Marshal(&result)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var raw map[string]json.RawMessage
+		if err := json.Unmarshal(data, &raw); err != nil {
+			t.Fatal(err)
+		}
+		if string(raw["inputRequests"]) != "{}" {
+			t.Errorf("empty InputRequests should marshal to {}, got %s", raw["inputRequests"])
+		}
+		var got CallToolResult
+		if err := json.Unmarshal(data, &got); err != nil {
+			t.Fatal(err)
+		}
+		if got.InputRequests == nil {
+			t.Error("empty InputRequests should round-trip as non-nil")
+		}
+	})
+
+	t.Run("populated round-trips", func(t *testing.T) {
+		result := CallToolResult{
+			Content: []Content{&TextContent{Text: "ok"}},
+			InputRequests: InputRequestMap{
+				"r1": &ElicitParams{Message: "confirm?"},
+			},
+		}
+		data, err := json.Marshal(&result)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var got CallToolResult
+		if err := json.Unmarshal(data, &got); err != nil {
+			t.Fatal(err)
+		}
+		if got.InputRequests == nil {
+			t.Fatal("InputRequests should not be nil after round-trip")
+		}
+		if _, ok := got.InputRequests["r1"]; !ok {
+			t.Error("expected key r1 in InputRequests")
+		}
+	})
+}
+
 func TestContentUnmarshal(t *testing.T) {
 	// Verify that types with a Content field round-trip properly.
 	roundtrip := func(in, out any) {