hive-agent: pin classify_tool_result's own 120-char boundary

Mutation testing found this arm untested: classify_tool_result has its
OWN `<= 120` deciding icon-vs-body, separate from the one in
summarize_tool_result that picks the summary text. The existing case
used 2 and 500 characters, so neither arm went near the boundary and
moving it to 119 changed nothing.

Verified the new test fails under exactly that mutation.

hive-agent/src/stream_enrich.rs

@@ -1283,6 +1283,24 @@ mod tests {
         })
     }
 
+    /// `classify_tool_result` has its **own** `<= 120`, separate from the one
+    /// in `summarize_tool_result`, and the two have to agree: one picks the
+    /// summary text, the other picks icon-vs-body. A result that summarises as
+    /// short but renders with a body is the inconsistency this pins.
+    /// Added because mutation testing found the gap — the case below uses 2
+    /// and 500 characters, so neither arm goes near the boundary and moving it
+    /// to 119 changed nothing.
+    #[test]
+    fn the_icon_or_body_choice_turns_on_the_same_120_boundary() {
+        let at = one(&tool_result(&"a".repeat(120), false, "t1"));
+        assert_eq!(at.icon.as_deref(), Some("←"), "120 chars is still short");
+        assert!(at.body.is_none());
+
+        let over = one(&tool_result(&"a".repeat(121), false, "t1"));
+        assert!(over.icon.is_none(), "121 chars stops being short");
+        assert_eq!(over.body.as_deref().map(str::len), Some(121));
+    }
+
     #[test]
     fn a_short_result_gets_an_icon_and_a_long_one_gets_a_body() {
         let short = one(&tool_result("ok", false, "t1"));