Fix undefined requestIDKey compilation error

Move contextKey type and requestIDKey constant to package level scope
so they can be accessed from multiple functions in the advanced example.
Remove duplicate definitions inside functions.

Author: vnykmshr

advanced

@@ -10,6 +10,11 @@ import (
 	"github.com/vnykmshr/obcache-go/pkg/obcache"
 )
 
+// Context key type for type safety
+type contextKey string
+
+const requestIDKey contextKey = "requestID"
+
 // User represents a user from the database
 type User struct {
 	ID       int       `json:"id"`
@@ -188,8 +193,6 @@ func main() {
 	cachedGetUserWithContext := obcache.Wrap(cache, userService.GetUserWithContext)
 
 	// Create contexts with request metadata
-	type contextKey string
-	const requestIDKey contextKey = "requestID"
 	ctx1 := context.WithValue(context.Background(), requestIDKey, "req-001")
 	ctx2 := context.WithValue(context.Background(), requestIDKey, "req-002")