Preserve images from last user message during context pruning #5575

Job	Run time
build-and-check	2m 43s
test-integration-packages/billing	1m 10s
test-integration-packages/internal	1m 3s
test-common	38s
test-integration-common	48s
test-.agents	43s
test-integration-sdk	39s
test-cli	1m 9s
test-integration-packages/agent-runtime	38s
test-packages/agent-runtime	57s
test-sdk	45s
test-integration-web	42s
test-web	39s
test-packages/billing	50s
test-packages/internal	50s
test-integration-cli	41s
	14m 55s

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