fix(summon): evict stale completed tasks to prevent unbounded memory growth (#9514)

Signed-off-by: Douwe M Osinga <douwe@sidewalklabs.com>
Co-authored-by: Douwe M Osinga <douwe@sidewalklabs.com>

crates/goose/src/agents/platform_extensions/summon.rs

@@ -80,6 +80,7 @@ pub struct CompletedTask {
     pub result: Result<String, String>,
     pub turns_taken: u32,
     pub duration: Duration,
+    pub completed_at: Instant,
 }
 
 #[derive(Debug, Deserialize)]
@@ -402,6 +403,13 @@ fn max_background_tasks() -> usize {
         .unwrap_or(5)
 }
 
+fn completed_task_ttl() -> Duration {
+    let secs = Config::global()
+        .get_param::<u64>("GOOSE_COMPLETED_TASK_TTL_SECS")
+        .unwrap_or(600);
+    Duration::from_secs(secs)
+}
+
 fn is_session_id(s: &str) -> bool {
     let parts: Vec<&str> = s.split('_').collect();
     parts.len() == 2 && parts[0].len() == 8 && parts[0].chars().all(|c| c.is_ascii_digit())
@@ -1560,9 +1568,13 @@ impl SummonClient {
                     result,
                     turns_taken,
                     duration,
+                    completed_at: Instant::now(),
                 },
             );
         }
+
+        let ttl = completed_task_ttl();
+        completed.retain(|_id, task| task.completed_at.elapsed() <= ttl);
     }
 
     fn get_task_description(params: &DelegateParams) -> String {
@@ -2491,6 +2503,7 @@ You review code."#;
                     result: Ok("Task completed successfully with output".to_string()),
                     turns_taken: 5,
                     duration: Duration::from_secs(60),
+                    completed_at: Instant::now(),
                 },
             );
             completed.insert(
@@ -2501,6 +2514,7 @@ You review code."#;
                     result: Err("Something went wrong".to_string()),
                     turns_taken: 3,
                     duration: Duration::from_secs(30),
+                    completed_at: Instant::now(),
                 },
             );
         }