fix(benchmarks): import CloudSandboxStrategy from cloud_aws; guard empty base_ref in benchmark-regression workflow

CloudSandboxStrategy is defined in cloud_aws.py, not cloud.py. The
benchmark was importing it from the wrong module, causing an ImportError
during benchmark discovery.

The benchmark-regression CI job's "Compute base commit" step used
forgejo.base_ref which is empty for push events, causing
`git merge-base HEAD "origin/"` to fail with exit 128. Add a guard
that falls back to HEAD~1 when base_ref is unset.

benchmarks/cloud_resource_bench.py

@@ -16,18 +16,18 @@ from pathlib import Path
 try:
     from cleveragents.resource.handlers.cloud import (
         CloudResourceHandler,
-        CloudSandboxStrategy,
         resolve_credentials,
         validate_credentials,
     )
+    from cleveragents.resource.handlers.cloud_aws import CloudSandboxStrategy
 except ModuleNotFoundError:
     sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
     from cleveragents.resource.handlers.cloud import (
         CloudResourceHandler,
-        CloudSandboxStrategy,
         resolve_credentials,
         validate_credentials,
     )
+    from cleveragents.resource.handlers.cloud_aws import CloudSandboxStrategy
 
 
 _CLOUD_ENV_VARS = [