Replace Linkup web search with Serper #6705

Job	Run time
build-and-check	2m 49s
test-integration-packages/internal	1m 13s
test-packages/agent-runtime	56s
test-common	47s
test-integration-cli	49s
test-integration-packages/agent-runtime	45s
test-packages/billing	44s
test-packages/internal	56s
test-integration-web	39s
test-cli	1m 4s
test-.agents	39s
test-integration-common	41s
test-integration-packages/billing	1m 8s
test-integration-sdk	2m 5s
test-sdk	45s
test-web	1m 7s
	17m 7s

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