This section details on the original issue you should resolve

<issue_title>OpenAPI Generation No Longer Groups Endpoints Using WithGroupName</issue_title>
<issue_description>### Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

In .NET 9, multiple Open API spec docs could be generated with a specific name utilizing the .WithGroupName extension method on either the group itself or particular endpoints. This allowed for a name other than the group's prefix to be used for grouping endpoints into an OpenAPI document that utilized a name other than the group's route prefix.

Now, it seems that the OpenAPI generator does not group those endpoints into the related spec document and instead excludes them entirely.

Expected Behavior

I should be able to group endpoints in an OpenAPI Spec document named something other than the Group prefix, utilizing .WithGroupName("MyGroupName") and .AddOpenApi("MyGroupName").

Steps To Reproduce

I have a minimal repro solution that demonstrates the behaviors here:
https://github.com/GoValidate/AspNetCore-Net10-OpenApi-Repro

The OpenApiNet9 and OpenApiNet10 are carbon copies of each other, with one targeting .NET 9 (and the .NET 9 version of AspNetCore OpenAPI) and the other targeting .NET 10 (and the .NET 10 version of AspNetCore OpenAPI). The code in both cases is the same:

var builder = WebApplication.CreateBuilder();

const string FirstGroup = "FirstGroup";
const string SecondGroup = "SecondGroup";

builder.Services.AddOpenApi();
builder.Services.AddOpenApi(FirstGroup);
builder.Services.AddOpenApi(SecondGroup);

var app = builder.Build();

app.MapOpenApi();

app.Map("no-group", () => "No group endpoint");
app.MapGroup("/first-group")
    .MapGet("/explicit", () => $".WithGroup(\"{FirstGroup}\") called on this endpoint")
    .WithGroupName(FirstGroup);

app.MapGroup("/second-group")
    .WithGroupName("SecondGroup")
    .MapGet("/implicit", () => $".WithGroup(\"{SecondGroup}\") called on the parent group");

app.Run();

New NET 10 Behavior

The following Open API docs are generated in NET 10:
https://localhost:5001/openapi/FirstGroup.json

{
  "openapi": "3.1.1",
  "info": {
    "title": "OpenApiNet10 | firstgroup",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "https://localhost:5001/"
    }
  ],
  "paths": { }
}

https://localhost:5001/openapi/SecondGroup.json

{
  "openapi": "3.1.1",
  "info": {
    "title": "OpenApiNet10 | secondgroup",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "https://localhost:5001/"
    }
  ],
  "paths": { }
}

Expected Behavior

In OpenApiNet9, the following swagger docs are generated:

https://localhost:7001/openapi/FirstGroup.json

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenApiNet9 | FirstGroup",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "https://localhost:7001/"
    }
  ],
  "paths": {
    "/first-group/explicit": {
      "get": {
        "tags": [
          "OpenApiNet9"
        ],
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "string"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": { },
  "tags": [
    {
      "name": "OpenApiNet9"
    }
  ]
}

https://localhost:7001/openapi/SecondGroup.json

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenApiNet9 | SecondGroup",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "https://localhost:7001/"
    }
  ],
  "paths": {
    "/second-group/implicit": {
      "get": {
        "tags": [
          "OpenApiNet9"
        ],
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "string"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": { },
  "tags": [
    {
      "name": "OpenApiNet9"
    }
  ]
}

Exceptions (if any)

No response

.NET Version

10.0.100

Anything else?

IDE is Rider (though this exhibits with dotnet run).

Output from dotnet --info

.NET SDK:
 Version:           10.0.100
 Commit:            b0f34d51fc
 Workload version:  10.0.100-manifests.4c0ca8ba
 MSBuild version:   18.0.2+b0f34d51f

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  26.1
 OS Platform: Darwin
 RID:         osx-arm64
 Base Path:   /usr/local/share/dotnet/sdk/10.0.100/

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
  Version:      10.0.0
  Architecture: arm64
  Commit:       b0f34d51fc

.NET SDKs installed:
  6...

</details>

- Fixes dotnet/aspnetcore#64335

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