fix(handoffs): avoid orphan reasoning items when nesting handoff history

[OiPunk]

Summary

• fix nest_handoff_history so standalone reasoning items are summarized but not forwarded as raw input items
• keep new_items unchanged for session persistence while filtering only next-agent model input
• add regression tests for reasoning filtering and tool-approval filtering paths

Why

With RunConfig(nest_handoff_history=True), forwarding a raw reasoning item can produce Responses API 400 errors (...reasoning... without its required following item).

Changes

• add "reasoning" to _SUMMARY_ONLY_INPUT_TYPES in src/agents/handoffs/history.py
• add tests in tests/test_extension_filters.py:
  • test_nest_handoff_history_does_not_forward_raw_reasoning_items
  • test_nest_handoff_history_skips_tool_approval_items
  • test_parse_summary_line_rejects_blank_lines

Validation

• uv run --with ruff ruff check src/agents/handoffs/history.py tests/test_extension_filters.py
• uv run --with mypy mypy src/agents/handoffs/history.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run --with pytest pytest tests/test_extension_filters.py -q
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run --with pytest pytest tests/test_run_step_processing.py -q
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run --with pytest --with pytest-cov pytest tests/test_extension_filters.py tests/test_run_step_processing.py --cov=agents.handoffs.history --cov-report=term-missing -q
  • coverage: src/agents/handoffs/history.py = 100%

Fixes #2503

[seratch]

Closing this in favor of #2508