Add pre-generation OpenAPI hook in AddOpenApi options to initialize document metadata

## Is there an existing issue for this?

- [X] I have searched the existing issues

## Describe the feature request

Please add a new AddOpenApi options hook that runs before operations/paths are generated, so callers can pre-populate the OpenApiDocument.

Today, AddDocumentTransformer runs after generation, which is too late for some scenarios.

Why this is needed

This would enable setting global tag metadata up front (for example richer Tag Object metadata in OpenAPI 3.2), including fields such as summary/description/parent/kind, then letting
generated operations reference those tags naturally.

Spec reference: https://spec.openapis.org/oas/v3.2.0.html#tag-object

Proposed API shape (example)

```csharp
builder.Services.AddOpenApi(options =>
{
options.AddPreGenerationDocumentHook((document, context, token) =>
{
document.Tags ??= [];
document.Tags.Add(new OpenApiTag
{
Name = "users",
Summary = "User operations",
Description = "Endpoints for user lifecycle."
// parent/kind when supported by the model
});
return Task.CompletedTask;
});
});
```

## Proposed execution order

1. Create base document (Info, Servers)
2. Run pre-generation hook
3. Generate paths/operations/components
4. Run existing document transformers

## Additional context

This would complement (not replace) AddDocumentTransformer, and improve support for advanced OpenAPI metadata composition.