Sign In fails when using custom OIDC provider

### Operating System

Ubuntu 22

### Environment (if applicable)

Chrome 132

### Firebase SDK Version

11.0.1

### Firebase SDK Product(s)

Auth

### Project Tooling

React 19 App With Vite

### Detailed Problem Description

We are developing an app in which we are trying to use Firebase's Authentication with a custom OIDC provider, but we are facing some errors when trying to sign in.
We have already followed the steps described in this guide: https://firebase.google.com/docs/auth/web/openid-connect

However, when we try to sign in, we receive the following error:

```
{
"error": {
"code": 400,
"message": "INVALID_IDP_RESPONSE : Error connecting to the given credential's issuer.",
"errors": [
{
"message": "INVALID_IDP_RESPONSE : Error connecting to the given credential's issuer.",
"domain": "global",
"reason": "invalid"
}
]
}
}
```

We have also tried to follow the troubleshooting guide available below, but none of the checks we executed indicated a problem.

https://cloud.google.com/iam/docs/troubleshooting-workforce-identity-federation#oidc_sign-in_errors

### Steps and code to reproduce issue

```
import { useCallback, useContext, useEffect } from "react";
import "./App.css";
import {
OAuthProvider,
getAuth,
getRedirectResult,
signInWithPopup,
signInWithRedirect,
} from "firebase/auth";

import FirebaseContext from "./FirebaseContext";

function App() {
const { authProvider, app } = useContext(FirebaseContext);

useEffect(() => {
getRedirectResult(getAuth())
.then((result) => {
if (!result) {
console.log("No redirect result from Firebase");
return;
}

// Get the OAuth access token and ID Token
const credential = OAuthProvider.credentialFromResult(result);
console.log("credential from firebase login redirect", credential);
})
.catch((error) => {
console.log("Error on Firebase getRedirectResult", error);
});
}, []);

const onLoginWithFirebasePopup = useCallback(async () => {
try {
const result = await signInWithPopup(getAuth(app), authProvider);
const credential = OAuthProvider.credentialFromResult(result);
console.log("user credential from firebase", credential);
} catch (error) {
console.error("Error on Firebase signInWithPopup", error);
}
}, [app, authProvider]);

const onLoginWithFirebaseRedirect = useCallback(async () => {
await signInWithRedirect(getAuth(), authProvider);
}, [authProvider]);

return (
<>

);
}

export default App;
```

Clicking on sign in button fails and redirect us back to our app.