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/mcp_test.go

@@ -24,6 +24,7 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/google/jsonschema-go/jsonschema"
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
@@ -206,7 +207,7 @@ func TestEndToEnd(t *testing.T) {
 					Role:    "user",
 				}},
 			}
-			if diff := cmp.Diff(wantReview, gotReview); diff != "" {
+			if diff := cmp.Diff(wantReview, gotReview, ctrCmpOpts...); diff != "" {
 				t.Errorf("prompts/get 'code_review' mismatch (-want +got):\n%s", diff)
 			}
 
@@ -2371,4 +2372,4 @@ func TestSetErrorPreservesContent(t *testing.T) {
 	}
 }
 
-var ctrCmpOpts = []cmp.Option{cmp.AllowUnexported(CallToolResult{})}
+var ctrCmpOpts = []cmp.Option{cmpopts.IgnoreUnexported(CallToolResult{}, GetPromptResult{}, ReadResourceResult{})}