Function open_diff 

pub async fn open_diff(
    bus: Option<&ProgrammaticDiffBus>,
    args: &Value,
    conn_id: u64,
    review: &ReviewHandle,
) -> Value

openDiff: open an interactive diff between old_file_path’s on-disk content and new_file_contents, blocking until the user resolves it.

Returns the full MCP CallToolResult envelope (NOT wrapped by the dispatch’s tool_text_result, since the content markers are the contract). Every failure path is graceful — missing args, an absent bus, or a dropped receiver all return a result rather than hanging or panicking.