fix: handle Mistral thinking content as reasoning chunks #14415

Job	Run time
platform-unit-test (windows-latest)	6m 36s
compile	2m 5s
knip	50s
platform-unit-test (ubuntu-latest)	3m 45s
check-translations	45s
check-openrouter-api-key	2s
integration-test	7m 53s
notify-slack-on-failure	0s
	21m 56s

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