fix: use system role for OpenAI Compatible provider when streaming is disabled #17390

Job	Run time
knip	46s
check-translations	40s
compile	2m 13s
platform-unit-test (ubuntu-latest)	4m 11s
platform-unit-test (windows-latest)	7m 14s
check-openrouter-api-key	3s
integration-test	7m 56s
	23m 3s

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