Replace Linkup web search with Serper #774

Job	Run time
build-freebuff	1m 14s
build-and-smoke-freebuff-windows	2m 47s
e2e-help-command	1m 11s
e2e-agent-startup	2m 20s
e2e-startup	1m 3s
e2e-version	1m 5s
e2e-code-edit	52s
e2e-terminal-command	51s
e2e-ads-behavior	1m 9s
e2e-slash-commands	1m 1s
	13m 33s

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