agent: fix misleading tokio::select! biased comment in do_read_stream

The previous comment incorrectly implied that `biased` prevents data
loss and the exit notifier would never be polled before all buffered
data is read. And the detailed info can be seen from the document:
https://docs.rs/tokio/latest/src/tokio/macros/select.rs.html#67

Tokio's `biased` only makes polling order deterministic(top-to-bottom)
when multiple branches are ready in the same poll, and it makes fairness
the caller's responsibility. Output can still be truncated if the exit
notification becomes ready while `read_stream` is pending.

This change updates the comment to reflect the actual semantics and
caveats. No functional behavior change.

Signed-off-by: Alex Lyn <alex.lyn@antgroup.com>

src/agent/src/rpc.rs

@@ -718,11 +718,17 @@ impl AgentService {
         // - Waker: https://doc.rust-lang.org/std/task/struct.Waker.html
         // - Tokio select!: https://docs.rs/tokio/latest/tokio/macro.select.html
         let data = tokio::select! {
-            // Poll the futures in the order they appear from top to bottom
-            // it is very important to avoid data loss. If there is still
-            // data in the buffer and read_stream branch will return
-            // Poll::Ready so that the term_exit_notifier will never polled
-            // before all data were read.
+            // Use `biased` to make the polling order deterministic (top-to-bottom).
+            // This ensures that *when multiple branches are ready at the same time*,
+            // we prefer reading pending output over reacting to the exit notification.
+            //
+            // Note: `biased` does NOT guarantee that we won't lose output. If the exit
+            // notification becomes ready while `read_stream` is still pending, the
+            // exit branch may be selected and we may stop reading before draining the
+            // remaining buffered data.
+            //
+            // Detailed information, please refer to Tokio doc for more information:
+            // https://docs.rs/tokio/latest/src/tokio/macros/select.rs.html#67
             biased;
 
             v = &mut read_fut => v?,