[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)

.github/workflows/rust-ci.yml

@@ -547,7 +547,7 @@ jobs:
   tests:
     name: Tests — ${{ matrix.runner }} - ${{ matrix.target }}${{ matrix.remote_env == 'true' && ' (remote)' || '' }}
     runs-on: ${{ matrix.runs_on || matrix.runner }}
-    timeout-minutes: 30
+    timeout-minutes: ${{ matrix.runner == 'windows-arm64' && 35 || 30 }}
     needs: changed
     if: ${{ needs.changed.outputs.codex == 'true' || needs.changed.outputs.workflows == 'true' || github.event_name == 'push' }}
     defaults: