fix(handoffs): preserve HandoffInputData.input_items in remove_all_tools

[adityasingh2400]

Summary

remove_all_tools rebuilds HandoffInputData via its constructor and silently drops the input_items field. The model-input pipeline (turn_resolution.py:484, :500) reads input_items when set — for example after nest_handoff_history populates it — so chained filters previously caused tool calls to leak into the next agent's input.

This switches to handoff_input_data.clone(...) (which preserves all fields) and also strips tool RunItems from input_items so the field remains consistent with the rest of the filtered data.

Repro

from agents.extensions.handoff_filters import nest_handoff_history, remove_all_tools

nested = nest_handoff_history(data)        # populates nested.input_items
filtered = remove_all_tools(nested)
assert filtered.input_items is not None    # fails on main, passes with fix

Test plan

• New unit test test_remove_all_tools_preserves_and_filters_input_items fails on main (AssertionError: assert None is not None) and passes with the fix.
• All 37 existing tests in tests/test_extension_filters.py continue to pass.
• tests/test_handoff_prompt.py continues to pass.