Preserve agent progress status throughout streamed turns

Problem

AppEvent::AgentMessageChunk clears tab.progress_status for every streamed message chunk. That can erase an agent-supplied ProgressStatus while the same turn is still active, even though the turn-state helpers describe progress as lasting until turn close or overwrite.

Expected behavior

Keep the current turn's progress status while chunks stream, and clear it only at the established turn boundaries or when a newer progress update replaces it.

Context

Found during review of #491: https://github.com/microsoft/intelligent-terminal/pull/491#discussion_r3644975519

The line existed in the original app.rs before #491 and was moved unchanged into app_events.rs; track the behavior fix separately so #491 remains a file-splitting-only PR.

Start by reading app_events.rs and the original handling in app.rs, then inspect the turn-state helpers that define progress lifetime. Trace AppEvent::AgentMessageChunk during a streamed turn and verify that progress remains until turn close or a newer update replaces it, while established boundaries still clear it.