Refactor env settings into config (#1601)

## Summary
- add OpenAI retry and timeout fields to Config
- inject these settings in tests instead of mutating env vars
- plumb Config values through client and chat completions logic
- document new configuration options

## Testing
- `cargo test -p codex-core --no-run`

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https://chatgpt.com/codex/tasks/task_i_68792c5b04cc832195c03050c8b6ea94

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Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/core/tests/cli_stream.rs

@@ -173,7 +173,7 @@ async fn integration_creates_and_checks_session_file() {
     // 5. Sessions are written asynchronously; wait briefly for the directory to appear.
     let sessions_dir = home.path().join("sessions");
     let start = Instant::now();
-    while !sessions_dir.exists() && start.elapsed() < Duration::from_secs(2) {
+    while !sessions_dir.exists() && start.elapsed() < Duration::from_secs(3) {
         std::thread::sleep(Duration::from_millis(50));
     }