Summary

Follow-up to #3394 (just merged) that fixed the text branch of `extract_last_content` against `None` from `model_construct` / provider gateway paths.

The refusal branch at `src/agents/items.py:693-694` has the same `-> str` return type contract and the same Pydantic-typed `str` source field (`ResponseOutputRefusal.refusal`), so it can violate the contract in the same way:

```python

from openai.types.responses import ResponseOutputMessage, ResponseOutputRefusal
from agents.items import ItemHelpers
refusal_part = ResponseOutputRefusal.model_construct(refusal=None, type="refusal")
msg = ResponseOutputMessage.model_construct(
... id="m", role="assistant", status="completed",
... type="message", content=[refusal_part])
ItemHelpers.extract_last_content(msg)
None # but the function is typed -> str
```

Apply the same `or ""` coercion and rationale comment that the text branch now has (items.py:687-691).

Scope (apologies for the noise on #3394)

Same scope as the merged version of #3394 — only the `-> str` branches get the coercion. `extract_last_text` (declared `-> str | None`) is intentionally left alone because:

• `None` is already a legal return per its signature.
• Coalescing `""` → `None` would silently change semantics for callers that distinguish empty text from missing text.

This PR is the equivalent fix on the refusal branch that I missed in #3394's first scope.

Test plan

• New `test_extract_last_content_returns_empty_string_for_none_refusal` — verifies the type contract holds for `refusal=None`.
• New `test_extract_last_content_returns_refusal_normally` — verifies regular refusal text still passes through unchanged.
• `uv run pytest tests/utils/test_pretty_print_and_items.py tests/test_items_helpers.py` — 43 passed.
• `uv run ruff check` on the touched files — All checks passed.

Issue number

N/A — direct follow-up to merged #3394.

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation (no doc changes needed)
• I've run `make lint` and `make format`
• I've made sure tests pass