fix(sandbox): normalize diagnostic tail limits

Signed-off-by: Paul Furgale <pfurgale@nvidia.com>

Author: furgalep

src/nooa/runtime/sandbox/serialization.py

@@ -25,7 +25,11 @@
 
 from nooa.agentdoc import TruncatingStringIO
 from nooa.config.truncation_config import DEFAULT_TRUNCATION_CONFIG
-from nooa.errors.formatting import _bound_preformatted_diagnostic, _hard_bound_text
+from nooa.errors.formatting import (
+    _bound_preformatted_diagnostic,
+    _diagnostic_budget,
+    _hard_bound_text,
+)
 from nooa.runtime.sandbox.errors import CellSerializationError
 
 # ``TruncatingStringIO`` adds a human-readable envelope around retained
@@ -53,7 +57,7 @@ def _bounded_error_message(
 ) -> str:
     """Apply the effective capture policy once to a raw message before IPC."""
     content_limit = _effective_error_limit(max_error)
-    effective_tail = tail_chars if tail_chars is None or tail_chars < content_limit else None
+    _, effective_tail = _diagnostic_budget(content_limit, tail_chars)
     if len(value) <= content_limit:
         return value
     stream = TruncatingStringIO(limit=content_limit, tail_chars=effective_tail)

tests/runtime/sandbox/test_error_serialization.py

@@ -199,6 +199,19 @@ def test_invalid_error_limit_falls_back_to_default(invalid_limit: object) -> Non
     assert "Showing first 5,000 and last 5,000 chars" in dto.error.message
 
 
+@pytest.mark.parametrize("invalid_tail", [-1, True])
+def test_invalid_error_tail_falls_back_to_valid_split(invalid_tail: object) -> None:
+    dto = result_to_dto(
+        _result_with_error(RuntimeError("x" * 1_000)),
+        max_error=100,
+        tail_chars=invalid_tail,  # type: ignore[arg-type]
+    )
+
+    assert dto.error is not None
+    assert "Showing first 50 and last 50 chars" in dto.error.message
+    assert dto.error.message.endswith("x" * 50 + "\n</truncated-output>")
+
+
 def test_error_dto_text_respects_configured_capture_limit_before_transport_limit() -> None:
     """Text above a custom content budget is truncated even if the DTO could carry it."""
     max_error = 100