Replace Linkup web search with Serper #773

Job	Run time
build-and-smoke-freebuff-windows	2m 57s
build-freebuff	1m 10s
e2e-terminal-command	57s
e2e-agent-startup	2m 10s
e2e-startup	1m 15s
e2e-slash-commands	59s
e2e-code-edit	51s
e2e-ads-behavior	1m 2s
e2e-help-command	1m 7s
e2e-version	1m 21s
	13m 49s

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