feat: allow per-call delegation model overrides

[adam91holt]

Summary

• add non-persistent model and provider args to delegate_task
• allow top-level delegation overrides plus per-task overrides in batch mode
• expose the new override fields in the delegate tool schema

Test Plan

• python -m pytest tests/tools/test_delegate.py -q -o 'addopts='
• fresh Hermes smoke: default delegated child used gpt-5.5, per-task override child used gpt-5.4-mini

[alt-glitch]

Duplicate of the per-call delegation model override cluster. Tracking issue: #14974. Open PRs implementing the same feature: #25813, #12794, #3172, #16163, #17756, #23649. Please consolidate onto one of the existing PRs.

[teknium1]

We do not want this