exec-server: tighten retained-output reads

Fix read pagination when max_bytes truncates a response, add a chunking regression covering stdout/stderr retention, warn on retained-output eviction, and note init auth as a pre-trust-boundary TODO.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/README.md

@@ -73,6 +73,9 @@ If the client sends exec methods before completing the `initialize` /
 If a connection closes, the server terminates any remaining managed processes
 for that connection.
 
+TODO: add authentication to the `initialize` setup before this is used across a
+trust boundary.
+
 ## API
 
 ### `initialize`