Allow creator sessions to answer child input requests

## Feature request

When a delegated agent session needs input, notify its creator and allow the creator to answer the pending request directly from the creator conversation.

Related orchestration work: #331521, foundation PR #331518, and notification PR #331544.

## Current behavior

The creator can be notified that a child needs input and can inspect the child's context. However, it cannot complete the pending input request from the creator session. Sending a normal message starts a separate turn and is not equivalent to answering the active request, so the user must open the child session to respond.

## Proposed behavior

Add a typed orchestration tool for responding to a child's active input request. It should:

- Target a specific child session and active request ID.
- Expose enough structured request context for the creator to form a valid response.
- Validate answers for the supported question type.
- Preserve explicit user confirmation for approvals or sensitive actions.
- Reject stale, completed, or mismatched requests.
- Respect the child's `coordinateWithCreator` policy.
- Work consistently across agent providers.

This would let a creator coordinate delegated work without leaving the parent conversation while avoiding the ambiguity and turn-state risks of treating a normal session message as an input response.