fix(delegate): save parent tool names before child construction mutates global

[ygd58]

Fixes #2079

Root Cause

The previous fix (#1804/#1894) moved _saved_tool_names into _run_single_child() to fix the NameError, but the save still captured the wrong value. _build_child_agent() calls AIAgent() which calls get_tool_definitions(), which overwrites model_tools._last_resolved_tool_names with the child's toolset. By the time _run_single_child() saves the global, it's already the child's tools — not the parent's.

Fix

1. Save parent tool names before any child construction in delegate_task() at the top of the build loop
2. Set correct value on each child via child._delegate_saved_tool_names = _parent_tool_names
3. Authoritative restore after all children are built: _model_tools._last_resolved_tool_names = _parent_tool_names
4. _run_single_child now reads from child._delegate_saved_tool_names (correct parent value)

Fixes Both Cases

• Single task: Parent tools correctly restored after delegation
• Batch mode: No race condition — each thread reads from its own child object, authoritative restore happens on main thread