feat(delegation): add model override to delegate_task (fixes #44900)

[Morad37]

Add an optional model parameter to delegate_task so subagents can run on a different model than the parent. Works like the existing model override in cronjob.

What changed

Function signature - delegate_task() now accepts model: Optional[Dict[str, str]] = None. Pass {provider: ..., model: ...} to override the model for all subagents.

Per-task override - Each task in the tasks array can also have its own model field, which beats the top-level override for that task only.

Resolution logic - Mirrors cronjob_tools._resolve_model_override. When you set model without provider, the current default provider is pinned at resolution time. This keeps the override stable even if you switch your default later.

Schema - Both the top-level and per-task model parameters are documented in the JSON schema as {type: object} with provider and model string properties.

Implementation details

• Added _resolve_model_override_for_delegate() helper, mirrors the cronjob version
• Top-level model is resolved after _resolve_delegation_credentials() and overrides the creds for all tasks
• Per-task model is resolved per child in the build loop and passed as model + override_provider to _build_child_agent()
• _build_child_agent() already accepted model and override_provider parameters, so no changes needed there

[alt-glitch]

Duplicate of #12715 — per-task/top-level model override for delegate_task. This is the earliest open PR in a saturated cluster (tracking #14974) of competing implementations (#17756, #41826, #31345). Consolidating there.

[tonydwb]

Code Review Summary

Verdict: Approved

Clean feature addition. The model parameter on delegate_task allows callers to override the model used by subagents, with proper per-task precedence (per-task overrides beat top-level, which beat delegation config).

Looks Good

• Implementation mirrors cronjob_tools._resolve_model_override for consistency
• When only model is specified without provider, pins the current default provider so the override doesn't drift on config change
• Schema properly documents the object structure with additionalProperties: False
• Top-level parameter and per-task model field are both exposed in the schema
• Clean error handling in _resolve_model_override_for_delegate with try/except

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Reviewed by Hermes Agent

[Morad37]

Agreed. #12715 covers this properly with the broader approach. Closing in favour of that. Thanks for the heads up @alt-glitch.