[APMSVLS-197] feat: Add fallback to extract trace context from event.request.headers

Add automatic trace context extraction from event.request.headers for
AppSync integration scenarios. This eliminates the need for customers
to use customized extractors for RUM → AppSync → Lambda resolver flows
where RUM-injected trace context is nested under event["request"]["headers"].

Author: Dogbu-cyber

bottlecap/src/lifecycle/invocation/processor.rs

@@ -1074,6 +1074,15 @@ impl Processor {
             return Some(sc);
         }
 
+        if let Some(request_obj) = payload_value.get("request") {
+            if let Some(request_headers) = request_obj.get("headers") {
+                if let Some(sc) = propagator.extract(request_headers) {
+                    debug!("Extracted trace context from event.request.headers");
+                    return Some(sc);
+                }
+            }
+        }
+
         None
     }