Coerce ToolCallItem/ToolCallOutputItem call_id to str | None consistently

[jluocsa]

Summary

ToolCallItem.call_id and ToolCallOutputItem.call_id returned the raw value untouched on the object branch, so a non-string call_id/id (e.g. an int) leaked through despite the str | None return annotation. This makes both properties consistently coerce to str | None, matching the canonical behavior of extract_tool_call_id in run_internal/tool_execution.py.

Changes

• ToolCallItem.call_id and ToolCallOutputItem.call_id now coerce the resolved value to str (or None when absent) on both the dict and object branches.

Test plan

• Added unit tests in tests/test_items_helpers.py:
  • dict and object raw items with string call_id return the string unchanged.
  • a non-string (int) call_id is coerced to "123".
  • a missing call_id returns None.
• make format, make lint, make typecheck, and the full make tests suite pass (4615 passed, 11 skipped).