fix: bias mini_agent serve() select toward shutdown over task exits

When shutdown_tx fires, each transport accept loop sees shutdown_rx,
drains its joinset, and returns Ok(()). Both the accept loop's
JoinHandle and the supervisor's shutdown_rx.changed() then become ready
in the same poll cycle. Without biased, the unbiased select! in serve()
would pick a *Died arm at random, return Err(...), and abort everything
without firing flusher_shutdown_tx — so the stats flusher's shutdown
branch never ran and no final stats POST happened. This made
test_mini_agent_*_with_real_flushers intermittently time out at 60s
waiting for /api/v0.2/stats; CI run 25562651282 captured the smoking
gun "Event::PipeDied: Ok(Ok(()))".

biased; with shutdown_rx.changed() listed first guarantees Event::Shutdown
wins whenever the shutdown signal is observable, so the supervisor always
takes the graceful-flush path even if the accept loops happen to resolve
first. Production main.rs never sends on shutdown_tx, so this is purely
a test-only path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Lewis-E

crates/datadog-trace-agent/src/mini_agent.rs

@@ -295,13 +295,19 @@ impl MiniAgent {
             };
             tokio::pin!(concentrator_exit);
 
+            // biased: when shutdown_tx fires, the accept loops also see
+            // shutdown_rx and exit cleanly. Their JoinHandles resolve in the
+            // same poll cycle as our shutdown_rx.changed(); without biased,
+            // a *Died arm can win the race and abort the supervisor before
+            // it has a chance to signal the stats flusher to flush.
             tokio::select! {
+                biased;
+                _ = shutdown_rx.changed() => Event::Shutdown,
                 r = &mut tcp_exit => Event::TcpDied(format!("{r:?}")),
                 r = &mut pipe_exit => Event::PipeDied(format!("{r:?}")),
                 r = &mut trace_flusher_handle => Event::TraceFlusherDied(format!("{r:?}")),
                 r = &mut stats_flusher_handle => Event::StatsFlusherDied(format!("{r:?}")),
                 r = &mut concentrator_exit => Event::ConcentratorDied(format!("{r:?}")),
-                _ = shutdown_rx.changed() => Event::Shutdown,
             }
         };
         #[cfg(not(all(windows, feature = "windows-pipes")))]
@@ -322,12 +328,14 @@ impl MiniAgent {
             };
             tokio::pin!(concentrator_exit);
 
+            // biased: see Windows branch above for rationale.
             tokio::select! {
+                biased;
+                _ = shutdown_rx.changed() => Event::Shutdown,
                 r = &mut tcp_exit => Event::TcpDied(format!("{r:?}")),
                 r = &mut trace_flusher_handle => Event::TraceFlusherDied(format!("{r:?}")),
                 r = &mut stats_flusher_handle => Event::StatsFlusherDied(format!("{r:?}")),
                 r = &mut concentrator_exit => Event::ConcentratorDied(format!("{r:?}")),
-                _ = shutdown_rx.changed() => Event::Shutdown,
             }
         };