feat: vertex/gemini prompt caching #5966

Job	Run time
check-translations	49s
compile	1m 15s
test-extension (ubuntu-latest)	2m 40s
test-extension (windows-latest)	3m 59s
check-openrouter-api-key	1s
knip	46s
test-webview (ubuntu-latest)	1m 31s
test-webview (windows-latest)	3m 11s
integration-test	0s
unit-test	1s
	14m 13s

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