hive-agent: build the multi-line test fixture without format_collect

clippy::format_collect (pedantic, denied) fires on
`(0..40).map(|i| format!(...)).collect::<String>()`. The test only needs
40 non-empty lines, not numbered ones, so `"line\n".repeat(40)` says the
same thing without the lint.

hive-agent/src/stream_enrich.rs

@@ -1249,7 +1249,7 @@ mod tests {
 
     #[test]
     fn a_long_tool_result_reports_its_line_count() {
-        let txt = (0..40).map(|i| format!("line {i}\n")).collect::<String>();
+        let txt = "line\n".repeat(40);
         let trimmed: String = txt.split_whitespace().collect::<Vec<_>>().join(" ");
         assert!(summarize_tool_result(&txt, &trimmed).starts_with("40L · "));
     }