The custom-agent example uses AsyncGenerator without importing it, which raises NameError when defining CustomAgent. Add the import and specify None as the send type so the annotation also evaluates on Python 3.12.

Validation:

• Reproduced the missing-import error and the single-type-argument error, then successfully defined CustomAgent after the change on Python 3.12, without model/API calls.
• Checked Markdown structure and git diff --check.
• Passed uv run --no-sync ruff format --check src tests examples, uv run --no-sync ruff check src tests examples, uv run --no-sync mypy, and uv run --no-sync ty check (example dependencies installed locally).

Suggested changelog label: internal (documentation only).