completions: handle body with no stream / codebuff_metadata #4075

Job	Run time
build-and-check	2m 54s
test-integration-.agents	44s
test-integration-backend	51s
test-integration-npm-app	47s
test-.agents	40s
test-integration-cli	46s
test-integration-web	51s
test-integration-common	50s
test-integration-packages/agent-runtime	47s
test-backend	46s
test-npm-app	47s
test-packages/agent-runtime	57s
test-cli	45s
test-web	50s
test-common	51s
	14m 6s

Provide feedback