Support session cost in copilot --acp

### Describe the feature or problem you'd like to solve

In version 1.0.78, copilot acp provides data for token usage and context windows size, without cost.

### Proposed solution

By any chance can we have the cost field in the session update object too?
https://agentclientprotocol.com/rfds/session-usage#cost-field

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start with the Copilot CLI implementation of ACP session updates and compare its existing token-usage and context-window data with the ACP session-usage cost-field specification linked in the issue. The work is done when session updates expose the requested cost data and the relevant behavior is covered by the project's tests.

Written by the indexing model from the issue text.