[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/app-server/tests/suite/v2/thread_start.rs

@@ -585,9 +585,10 @@ request_max_retries = 0
 stream_max_retries = 0
 
 [mcp_servers.required_broken]
-command = "codex-definitely-not-a-real-binary"
+{required_broken_transport}
 required = true
-"#
+"#,
+            required_broken_transport = broken_mcp_transport_toml()
         ),
     )
 }
@@ -615,8 +616,21 @@ request_max_retries = 0
 stream_max_retries = 0
 
 [mcp_servers.optional_broken]
-command = "codex-definitely-not-a-real-binary"
-"#
+{optional_broken_transport}
+"#,
+            optional_broken_transport = broken_mcp_transport_toml()
         ),
     )
 }
+
+#[cfg(target_os = "windows")]
+fn broken_mcp_transport_toml() -> &'static str {
+    r#"command = "cmd"
+args = ["/C", "exit 1"]"#
+}
+
+#[cfg(not(target_os = "windows"))]
+fn broken_mcp_transport_toml() -> &'static str {
+    r#"command = "/bin/sh"
+args = ["-c", "exit 1"]"#
+}