acp: Suggest installing gemini@preview instead of latest (#36629)

Release Notes:

- N/A

crates/agent_servers/src/gemini.rs

@@ -53,7 +53,7 @@ impl AgentServer for Gemini {
                 return Err(LoadError::NotInstalled {
                     error_message: "Failed to find Gemini CLI binary".into(),
                     install_message: "Install Gemini CLI".into(),
-                    install_command: "npm install -g @google/gemini-cli@latest".into()
+                    install_command: "npm install -g @google/gemini-cli@preview".into()
                 }.into());
             };