Function drain_and_collect 

async fn drain_and_collect(
    stdout: ChildStdout,
    tx: &UnboundedSender<AgentEvent>,
    child: &mut Child,
) -> Result<AgentRunResult, String>

Drain stdout line-by-line through ClaudeTranslator, forward every emitted event on tx, accumulate the terminal Cost / Done event payloads into an AgentRunResult, wait for the child to exit, and return the result.

Split out from run_agent so it can be unit-tested against in-memory readers without spawning a real subprocess. Used by the integration test below as drain_async_reader_for_test.