[APMSVLS-197] Add check to extract trace context present within event.request.headers (#1011)

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

## Overview

This PR adds 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"]`.

## Problem

Previously, trace context from RUM was not propagated through AppSync to Lambda resolvers because the extension only checked:
1. Trigger-specific headers (via span inferrer)
2. `event.headers` (top-level event headers)
3. Request headers (HTTP request headers)

For AppSync, the trace context is nested under `event.request.headers`, which was not checked, causing traces to break at the AppSync boundary.

## Solution

Added a step that checks `event.request.headers` after all other trace context sources have been attempted.

## Trace Examples

### Before 
Without this change, trace context is lost at the AppSync boundary:

[Trace showing broken propagation through AppSync](https://ddserverless.datadoghq.com/apm/trace/5616290163800553628?graphType=waterfall&shouldShowLegend=true&spanID=5616290163800553628&timeHint=1769698078685.9998&traceQuery=)

[Log showing the passed header value for x-datadog-trace-id does not align with the given trace_id](https://ddserverless.datadoghq.com/logs?query=%40lambda.request_id%3A%28cba225c5-7b76-45d8-9a43-86df9031687f%29%20OR%20trace_id%3A5616290163800553628&agg_m=count&agg_m_source=base&agg_t=count&cols=host%2Cservice&event=AwAAAZwKObH6tpUUmwAAABhBWndLT2JqNEFBQWpWdUZHSkF5YzFRQXgAAAAkZjE5YzBhNDQtYjE3Zi00YjljLTk2NzMtNWY2NzI2OTc2MDlmAA2mrg&fromUser=true&messageDisplay=inline&refresh_mode=paused&storage=flex_tier&stream_sort=desc&viz=stream&from_ts=1769697078981&to_ts=1769699079405&live=false)


Flow: `RUM → AppSync → Lambda` (trace ID not propagated)

### After
With this change, trace context is properly extracted and propagated:

[Trace showing successful propagation through AppSync](https://ddserverless.datadoghq.com/apm/trace/12345?graphType=waterfall&shouldShowLegend=true&spanID=7582932037133874185&timeHint=1769698399447&traceQuery=)

Flow: `RUM → AppSync → Lambda resolver` (trace continues using value within x-datadog-trace-id)

## Testing

### Unit Tests

Added 10 unit tests covering:

- Extraction from `event.request.headers` with Datadog headers
- Extraction from `event.request.headers` with W3C TraceContext headers
- Edge cases (missing headers, empty objects, invalid data)

Run tests with:
```bash
cargo test test_extract_span_context -- --nocapture
```



[APMSVLS-197]: https://datadoghq.atlassian.net/browse/APMSVLS-197?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Co-authored-by: david.ogbureke <[email protected]>

Author: Dogbu-cyber

bottlecap/src/lifecycle/invocation/processor.rs

@@ -1076,6 +1076,15 @@ impl Processor {
             return Some(sc);
         }
 
+        if let Some(sc) = payload_value
+            .get("request")
+            .and_then(|req| req.get("headers"))
+            .and_then(|headers| propagator.extract(headers))
+        {
+            debug!("Extracted trace context from event.request.headers");
+            return Some(sc);
+        }
+
         if let Some(payload_headers) = payload_value.get("headers") {
             if let Some(sc) = propagator.extract(payload_headers) {
                 debug!("Extracted trace context from event headers");
@@ -1851,4 +1860,71 @@ mod tests {
             "Should be snapstart span (id=3), not cold start span (id=2)"
         );
     }
+
+    #[test]
+    fn test_extract_span_context_priority_order() {
+        let config = Arc::new(config::Config {
+            trace_propagation_style_extract: vec![
+                config::trace_propagation_style::TracePropagationStyle::Datadog,
+                config::trace_propagation_style::TracePropagationStyle::TraceContext,
+            ],
+            ..config::Config::default()
+        });
+        let propagator = Arc::new(DatadogCompositePropagator::new(Arc::clone(&config)));
+
+        let mut headers = HashMap::new();
+        headers.insert(DATADOG_TRACE_ID_KEY.to_string(), "111".to_string());
+        headers.insert(DATADOG_PARENT_ID_KEY.to_string(), "222".to_string());
+
+        let payload = json!({
+            "headers": {
+                "x-datadog-trace-id": "333",
+                "x-datadog-parent-id": "444"
+            },
+            "request": {
+                "headers": {
+                    "x-datadog-trace-id": "555",
+                    "x-datadog-parent-id": "666"
+                }
+            }
+        });
+
+        let result = Processor::extract_span_context(&headers, &payload, propagator);
+
+        assert!(result.is_some());
+        let context = result.unwrap();
+        assert_eq!(
+            context.trace_id, 555,
+            "Should prioritize event.request.headers as service-specific extraction"
+        );
+    }
+
+    #[test]
+    fn test_extract_span_context_no_request_headers() {
+        let config = Arc::new(config::Config {
+            trace_propagation_style_extract: vec![
+                config::trace_propagation_style::TracePropagationStyle::Datadog,
+                config::trace_propagation_style::TracePropagationStyle::TraceContext,
+            ],
+            ..config::Config::default()
+        });
+        let propagator = Arc::new(DatadogCompositePropagator::new(Arc::clone(&config)));
+        let headers = HashMap::new();
+
+        let payload = json!({
+            "argumentsMap": {
+                "id": "123"
+            },
+            "request": {
+                "body": "some body"
+            }
+        });
+
+        let result = Processor::extract_span_context(&headers, &payload, propagator);
+
+        assert!(
+            result.is_none(),
+            "Should return None when no trace context found"
+        );
+    }
 }