Restore MultiAgentV2 child configuration and lifecycle contracts

[shiny-code-bot]

Objective

Restore the intended MultiAgentV2 child configuration, reload, and lifecycle behavior exposed by the largest coherent failure cluster in release run 31973347222.

Finish Line

Spawned and reloaded children preserve the correct stored/requested model, provider, reasoning effort, role, runtime sandbox, and direct-input capability; send-input and lifecycle notifications operate against that child identity; app-server activity and thread-list projections agree with the runtime. The repair does not restore MultiAgentV1 behavior or rewrite existing model-visible history.

Current Status

Acceptance Criteria

• Reloaded registered children retain their stored model/provider/reasoning contract.
• Requested defaults, configured defaults, roles, and runtime overrides apply in documented precedence order.
• Cross-backend and unsupported-effort rejection remains fail-closed.
• send_input targets the correct child and preserves interrupt-before-prompt semantics.
• App-server spawn activity and thread-list direct-input capability match runtime state.
• Focused unit and integration tests pass without broad snapshot acceptance.
• The change remains under 500 lines or is split before review.

Relationships

• Sub-issue of Restore lab.5 release Rust test convergence #703.
• Highest-leverage product repair after Clear first deterministic lab.5 release-test blockers #704.
• Failure source: validation-only release run 31973347222 at main@70c94978062060230a658a2dfbc08008c42d18a9.