Replace Linkup web search with Serper #6701

Job	Run time
build-and-check	2m 57s
test-integration-packages/agent-runtime	42s
test-integration-common	42s
test-integration-packages/internal	1m 15s
test-integration-web	42s
test-sdk	40s
test-integration-cli	43s
test-integration-sdk	1m 44s
test-packages/billing	48s
test-common	43s
test-packages/agent-runtime	44s
test-web	53s
test-integration-packages/billing	1m 1s
test-packages/internal	45s
test-cli	49s
test-.agents	42s
	15m 50s

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