fix: change admin key validation from error to warning when not provided (#59)

* fix: change admin key validation from error to warning when not provided

Previously, attempting to configure the OpenAI provider with a null admin_key would result in an error, making it impossible to use configurations where the admin key might be intentionally unset for certain environments or accounts.

This change:
- Changes the admin_key validation from an error to a warning when not provided
- Only performs the sk-admin- prefix validation when an admin_key is actually provided
- Enables common configurations like:
  ```hcl
  provider openai {
    admin_key = var.openai_api_key # where var.openai_api_key can be null
  }

* Update internal/provider/provider.go

Co-authored-by: Edd <edward.stephinson@gmail.com>

---------

Co-authored-by: Edd <edward.stephinson@gmail.com>

Author: rorymalcolm

internal/provider/provider.go

@@ -80,8 +80,7 @@ func (p *OpenAIProvider) Configure(ctx context.Context, req provider.ConfigureRe
 	}
 
 	if adminKey == "" {
-		resp.Diagnostics.AddError("admin_key is required", "admin_key is required")
-		return
+		resp.Diagnostics.AddWarning("admin_key is required", "admin_key is required")
 	} else if !strings.HasPrefix(adminKey, "sk-admin-") {
 		resp.Diagnostics.AddError("admin_key must start with 'sk-admin-'", "admin_key must start with 'sk-admin-'")
 		return