ACP: tool_call_update in_progress notifications don't include tool metadata #14592

Description

When a tool (e.g. the task tool) calls ctx.metadata() during execution, the metadata is persisted on the part via Session.updatePart() and a PartUpdated event fires. However, the ACP agent only includes metadata in the completed tool_call_update notification — in_progress notifications omit it entirely.

This is relevant for the task tool specifically: it calls ctx.metadata({ sessionId: session.id, ... }) immediately after creating the child session, before the subagent begins execution. The ToolStateRunning schema already supports an optional metadata field, and the data is available on part.state.metadata, but src/acp/agent.ts doesn't forward it in the running case.

Impact

ACP clients that want to display or interact with a running subagent session cannot obtain the child sessionId until the subagent finishes. This prevents use cases like live session peeking / tailing a subagent while it runs.

Expected behavior

tool_call_update notifications with status: "in_progress" should include rawOutput.metadata when the tool has set metadata via ctx.metadata(), matching the structure already used in completed notifications.