Add unit tests for response-terminal failure formatting

[jluocsa]

Summary

src/agents/models/_response_terminal.py builds the human-readable error messages used when a Responses stream ends on a terminal/error event. It was at 86% coverage: the detail-aggregation branches (status/error/incomplete_details and code/message/param) and the two error-wrapping helpers were not directly exercised. This adds focused unit tests that bring the module to 100% without touching any runtime code.

Changes

• New tests/models/test_response_terminal.py covering:
  • format_response_terminal_failure with no response, all details present, a mix of falsy/non-falsy details, and no details.
  • format_response_error_event with all details, partial details, and no details.
  • response_terminal_failure_error and response_error_event_failure_error wrapping the formatted message in a ModelBehaviorError.

Test plan

• uv run pytest tests/models/test_response_terminal.py passes (9 tests).
• _response_terminal.py coverage rises from 86% to 100%.
• ruff format, ruff check, and mypy are clean on the new file.