ci: warn-and-fall-through when no upstream ddtrace wheel matches

dd-trace-py 4.x stopped building cp38 wheels (PYTHON_TAGS in
.gitlab/package.yml starts at cp39), so when this repo's CI is triggered
from a dd-trace-py 4.x PR, the cp38 unit-test job's spec_ddtrace_dep
call hard-errored. Lambda-python's pyproject.toml already pins
ddtrace<4 for Python 3.8/3.9, so the released ddtrace is the correct
thing to use there.

Match the original build_layers.sh behavior: warn, leave pyproject.toml
untouched, return 0. The dd-trace-py upload-serverless job's status
independently signals build/upload failures for supported versions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: purple4reina

scripts/_spec_ddtrace_dep.sh

@@ -75,8 +75,15 @@ spec_ddtrace_dep() {
         else
             PLATFORM="manylinux2014_aarch64"
         fi
-        _search_and_spec_s3_wheel "ddtrace_serverless" "serverless" \
-            || _search_and_spec_s3_wheel "ddtrace" "manylinux2014" \
-            || { echo "ERROR: No matching ddtrace wheel for ${PY_TAG} ${PLATFORM} in pipeline ${UPSTREAM_PIPELINE_ID}" >&2; return 1; }
+        # Fall through to the released ddtrace when no wheel matches. This
+        # is expected for Python versions that dd-trace-py no longer builds
+        # for (e.g. cp38 on dd-trace-py 4.x), and matches the original
+        # build_layers.sh "No S3 wheel found, using default pyproject.toml
+        # version" behavior. The dd-trace-py upload-serverless job's status
+        # independently signals build/upload failures for supported versions.
+        if ! _search_and_spec_s3_wheel "ddtrace_serverless" "serverless" \
+            && ! _search_and_spec_s3_wheel "ddtrace" "manylinux2014"; then
+            echo "WARNING: No matching ddtrace wheel for ${PY_TAG} ${PLATFORM} in pipeline ${UPSTREAM_PIPELINE_ID} — falling back to released ddtrace from pyproject.toml" >&2
+        fi
     fi
 }