Fix duplicate aws.lambda spans when extension is present

When the extension is present, use the placeholder resource name
"dd-tracer-serverless-span" for the Python tracer's aws.lambda span.
This allows the extension to:
1. Recognize the span as a tracer placeholder
2. Merge the span's metadata into the extension's invocation span
3. Filter out the placeholder span (not sent to backend)

This prevents duplicate aws.lambda spans being sent - only the extension's
span (with merged Python metadata) will be sent.

Without the extension, the span uses the function name as before.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jchrostek-dd

datadog_lambda/tracing.py

@@ -35,6 +35,7 @@
 from ddtrace.trace import Context, Span, tracer
 
 from datadog_lambda.config import config
+from datadog_lambda.extension import should_use_extension
 from datadog_lambda import __version__ as datadog_lambda_version
 from datadog_lambda.trigger import (
     _EventSource,
@@ -1493,10 +1494,18 @@ def create_function_execution_span(
         else:
             service_name = function_name if function_name else "aws.lambda"
 
+    # When the extension is present, use a placeholder resource name so the
+    # extension recognizes this span and merges its metadata into the extension's
+    # invocation span. The extension will set the correct resource (function name)
+    # on its span, and this placeholder span will be filtered out.
+    # Without the extension, use the function name directly.
+    span_resource = (
+        "dd-tracer-serverless-span" if should_use_extension else function_name
+    )
     span = tracer.trace(
         "aws.lambda",
         service=service_name,
-        resource=function_name,
+        resource=span_resource,
         span_type="serverless",
     )
     if span: