[codex-cli][app-server] Update self-serve business usage limit copy in error returned (#15478)

## Summary
- update the self-serve business usage-based limit message to direct
users to their admin for additional credits
- add a focused unit test for the self_serve_business_usage_based plan
branch

Added also: 

If you are at a rate limit but you still have credits, codex cli would
tell you to switch the model. We shouldnt do this if you have credits so
fixed this.

## Test
- launched the source-built CLI and verified the updated message is
shown for the self-serve business usage-based plan

![Test
screenshot](https://raw.githubusercontent.com/openai/codex/5cc3c013ef17ac5c66dfd9395c0d3c4837602231/docs/images/self-serve-business-usage-limit.png)

codex-rs/core/tests/suite/shell_snapshot.rs

@@ -72,7 +72,7 @@ async fn wait_for_snapshot(codex_home: &Path) -> Result<PathBuf> {
 }
 
 async fn wait_for_file_contents(path: &Path) -> Result<String> {
-    let deadline = Instant::now() + Duration::from_secs(5);
+    let deadline = Instant::now() + Duration::from_secs(15);
     loop {
         match fs::read_to_string(path).await {
             Ok(contents) => return Ok(contents),
@@ -575,7 +575,32 @@ async fn shell_command_snapshot_still_intercepts_apply_patch() -> Result<()> {
     let snapshot_content = fs::read_to_string(&snapshot_path).await?;
     assert_posix_snapshot_sections(&snapshot_content);
 
-    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
+    let mut saw_patch_begin = false;
+    let mut patch_end = None;
+    wait_for_event(&codex, |ev| match ev {
+        EventMsg::PatchApplyBegin(begin) if begin.call_id == call_id => {
+            saw_patch_begin = true;
+            false
+        }
+        EventMsg::PatchApplyEnd(end) if end.call_id == call_id => {
+            patch_end = Some(end.clone());
+            false
+        }
+        EventMsg::TurnComplete(_) => true,
+        _ => false,
+    })
+    .await;
+
+    assert!(
+        saw_patch_begin,
+        "expected apply_patch to emit PatchApplyBegin"
+    );
+    let patch_end = patch_end.expect("expected apply_patch to emit PatchApplyEnd");
+    assert!(
+        patch_end.success,
+        "expected apply_patch to finish successfully: stdout={:?} stderr={:?}",
+        patch_end.stdout, patch_end.stderr,
+    );
 
     assert_eq!(
         wait_for_file_contents(&target).await?,