Use step settings for MCP request metadata (#40807)

## Why

MCP tool calls can be issued by a model step whose settings differ from the
turn's initial settings. Request metadata must describe the issuing step.

## What changed

- Build MCP tool-call metadata from the captured `StepContext` model and its
  effective reasoning effort.
- Populate `node_repl_disabled` from the issuing step while leaving turn-owned
  Responses metadata unchanged.
- Apply the same step-scoped values to MCP requests emitted by stop hooks.

## Testing

Add coverage for configured and model-default reasoning effort, model changes,
Node REPL restrictions, and preservation of the original step and turn
metadata.

GitOrigin-RevId: f9cce52fbe559f15437bd19d699b657229c0d145