feat: Fairly trim skill descriptions within context budget (#18925)

Preserve skill name/path entries whenever possible and trim descriptions
first, using round-robin character allocation so short descriptions do
not waste budget.

codex-rs/tui/src/chatwidget/tests/app_server.rs

@@ -191,7 +191,7 @@ async fn live_app_server_warning_notification_renders_message() {
     chat.handle_server_notification(
         ServerNotification::Warning(WarningNotification {
             thread_id: None,
-            message: "Some enabled skills were not included in the model-visible skills list for this session. Mention a skill by name or path if you need it.".to_string(),
+            message: "Warning: Exceeded skills context budget of 2%. All skill descriptions were removed and 2 additional skills were not included in the model-visible skills list.".to_string(),
         }),
         /*replay_kind*/ None,
     );
@@ -201,13 +201,13 @@ async fn live_app_server_warning_notification_renders_message() {
     let rendered = lines_to_single_string(&cells[0]);
     let normalized = rendered.split_whitespace().collect::<Vec<_>>().join(" ");
     assert!(
-        normalized.contains(
-            "Some enabled skills were not included in the model-visible skills list for this session."
-        ),
+        normalized.contains("Warning: Exceeded skills context budget of 2%."),
         "expected warning notification message, got {rendered}"
     );
     assert!(
-        normalized.contains("Mention a skill by name or path if you need it."),
+        normalized.contains(
+            "All skill descriptions were removed and 2 additional skills were not included in the model-visible skills list."
+        ),
         "expected warning guidance, got {rendered}"
     );
 }