`UsePathBase()` not being taking into account when generating OpenApi document with `AddOpenApi()`

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

I have currently an ASP.NET Web API in .NET 9.0.4 (SDK 9.0.203) in which I use the OpenApi document generator calling `AddOpenApi()` in the services pipeline (with `Microsoft.AspNetCore.OpenApi`).

After calling the method `AddOpenApi()` I map a custom base path via `app.UsePathBase("/myapi");` and after that I call `app.MapOpenApi();` to map the path to serve the OpenApi document.

Nevertheless, It does not take this `PathBase` into account in the generated OpenApi doc (served in my case in `/openapi/v1.json`) in the servers array in the URL field. For example:

```json
{
"openapi": "3.0.1",
"info": {
"title": "Api | v1",
"version": "1.0.0"
},
"servers": [
{
"url": "https://example.org/"
}
],
"paths": {
"/v1/test": {
}
}
...
}
```

### Expected Behavior

The expected behaviour would be that the `servers.url` field would contain that base path if specified (since `basePath` property in openapi is deprecated in its version 3.x).

### Steps To Reproduce

Simply by following the given example here: https://devblogs.microsoft.com/dotnet/dotnet9-openapi/ and adding a call to establish a path base like: `app.UsePathBase("/myapi");`

### Exceptions (if any)

_No response_

### .NET Version

9.0.203

### Anything else?

ASP.NET Core version: Runtime 9.0.4
Current IDE: Visual Studio 2022 (Professional)
`dotnet --info` output:

```
SDK DE .NET:
Version: 9.0.203
Commit: dc7acfa194
Workload version: 9.0.200-manifests.12d79ccf
MSBuild version: 17.13.20+a4ef1e90f

Entorno de tiempo de ejecución:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.203\

Cargas de trabajo de .NET instaladas:
No hay cargas de trabajo instaladas para mostrar.
Configurado para usar loose manifests al instalar nuevos manifiestos.

Host:
Version: 9.0.4
Architecture: x64
Commit: f57e6dc747

.NET SDKs installed:
9.0.203 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
```

Start by reproducing the documented AddOpenApi(), UsePathBase("/myapi"), and MapOpenApi() setup from the issue. Trace how the generated document sets servers.url, then verify that the result includes /myapi while preserving the existing OpenAPI paths and document endpoint behavior.

Written by the indexing model from the issue text.