perf(agent): avoid preview allocations for non-truncated strings (fix #894) (#924)

* perf(agent): avoid preview allocation on non-truncated strings

* Update src/worker/container.rs

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* chore(ci): annotate test assertions for no-panics gate

* fix: remove unnecessary allocation and consolidate tests

- Remove redundant `.to_string()` on `&String` in container.rs error arm
- Bind `format!()` result to a let in job.rs to avoid Cow borrowing from temporary
- Merge borrowed/owned Cow assertions into existing tests, drop misleading comments

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix: restore separate test functions for CI regression check

Keep dedicated `test_truncate_short_string_borrows` and
`test_truncate_long_string_owns` tests so the PR diff contains
new `#[test]` functions, satisfying the regression test enforcement check.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: [email protected] <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

src/agent/agentic_loop.rs

@@ -6,6 +6,7 @@
 //! via the `LoopDelegate` trait.
 
 use async_trait::async_trait;
+use std::borrow::Cow;
 
 use crate::agent::session::PendingApproval;
 use crate::error::Error;
@@ -235,12 +236,12 @@ pub async fn run_agentic_loop(
 ///
 /// `max` is a byte budget. The result is truncated at the last valid char
 /// boundary at or before `max` bytes, so it is always valid UTF-8.
-pub fn truncate_for_preview(s: &str, max: usize) -> String {
+pub fn truncate_for_preview(s: &str, max: usize) -> Cow<'_, str> {
     if s.len() <= max {
-        s.to_string()
+        Cow::Borrowed(s)
     } else {
         let end = crate::util::floor_char_boundary(s, max);
-        format!("{}...", &s[..end])
+        Cow::Owned(format!("{}...", &s[..end]))
     }
 }
 
@@ -597,12 +598,24 @@ mod tests {
         assert_eq!(truncate_for_preview("hello", 10), "hello");
     }
 
+    #[test]
+    fn test_truncate_short_string_borrows() {
+        let result = truncate_for_preview("hello", 10);
+        assert!(matches!(result, Cow::Borrowed("hello")));
+    }
+
     #[test]
     fn test_truncate_long_string_adds_ellipsis() {
         let result = truncate_for_preview("hello world", 5);
         assert_eq!(result, "hello...");
     }
 
+    #[test]
+    fn test_truncate_long_string_owns() {
+        let result = truncate_for_preview("hello world", 5);
+        assert!(matches!(result, Cow::Owned(_)));
+    }
+
     #[test]
     fn test_truncate_multibyte_safe() {
         let result = truncate_for_preview("café", 4);

src/worker/container.rs

@@ -472,7 +472,7 @@ impl LoopDelegate for ContainerDelegate {
                     "tool_name": tc.name,
                     "output": match &result {
                         Ok(output) => truncate_for_preview(output, 2000),
-                        Err(e) => format!("Error: {}", truncate_for_preview(e, 500)),
+                        Err(e) => format!("Error: {}", truncate_for_preview(e, 500)).into(),
                     },
                     "success": result.is_ok(),
                 }),

src/worker/job.rs

@@ -800,12 +800,16 @@ Report when the job is complete or if you encounter issues you cannot resolve."#
                     });
                 }
 
+                let error_preview = {
+                    let msg = format!("Error: {}", e);
+                    truncate_for_preview(&msg, 500).into_owned()
+                };
                 self.log_event(
                     "tool_result",
                     serde_json::json!({
                         "tool_name": selection.tool_name,
                         "success": false,
-                        "output": truncate_for_preview(&format!("Error: {}", e), 500),
+                        "output": error_preview,
                     }),
                 );