mcp: preserve structured content number precision (#1239)

CallToolResult currently decodes structuredContent through an any field,
which converts JSON numbers to float64 and silently rounds integers
outside the IEEE-754 safe range.

This change decodes structuredContent separately with UseNumber,
preserving wire numbers as json.Number while retaining the existing
decoded object, array, and primitive shapes. The field documentation now
describes that representation. Users who depend on the previous float64
behavior can temporarily restore it with
MCPGODEBUG=structuredcontentfloat64=1.

An end-to-end in-memory client/server regression covers 9007199254740993
from a low-level Server.AddTool result. Additional coverage verifies the
compatibility option restores the legacy rounded float64 value.

Verification:
- gofmt -l .
- go test ./...
- go vet ./...

Fixes #1236

---------

Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/protocol_test.go

@@ -1285,7 +1285,7 @@ func TestContentUnmarshal(t *testing.T) {
 		Meta:              Meta{"m": true},
 		Content:           content,
 		IsError:           true,
-		StructuredContent: 3.0,
+		StructuredContent: json.Number("3"),
 	}
 	var gotf CallToolResult
 	roundtrip(ctrf, &gotf)
@@ -1298,6 +1298,24 @@ func TestContentUnmarshal(t *testing.T) {
 	roundtrip(pm, &gotpm)
 }
 
+func TestCallToolResultStructuredContentFloat64Compatibility(t *testing.T) {
+	prev := structuredcontentfloat64
+	structuredcontentfloat64 = "1"
+	t.Cleanup(func() { structuredcontentfloat64 = prev })
+
+	var got CallToolResult
+	if err := json.Unmarshal([]byte(`{"content":[],"structuredContent":{"id":9007199254740993}}`), &got); err != nil {
+		t.Fatal(err)
+	}
+	structured, ok := got.StructuredContent.(map[string]any)
+	if !ok {
+		t.Fatalf("StructuredContent type = %T, want map[string]any", got.StructuredContent)
+	}
+	if got := structured["id"]; got != float64(9007199254740992) {
+		t.Errorf("id = %v (%T), want rounded float64 value", got, got)
+	}
+}
+
 func TestToolAnnotations_MarshalJSON(t *testing.T) {
 	boolPtr := func(b bool) *bool { return &b }