Test display custom fallback truncation semantics

Co-authored-by: Shri Sukhani <shrisukhani@users.noreply.github.com>

Author: cursoragent

tests/test_display_utils.py

@@ -115,3 +115,14 @@ class _StringSubclass(str):
         )
         == "<blank key>"
     )
+
+
+def test_format_string_key_for_error_truncates_custom_blank_fallback():
+    assert (
+        format_string_key_for_error(
+            "   ",
+            max_length=12,
+            blank_fallback="fallback-value-too-long",
+        )
+        == "... (truncated)"
+    )