Add phone notification template support to terraform generation

[bkiran6398]

🔧 Changes

Adds auth0_phone_notification_template resource support to the auth0 tf generate command.

• Introduces PhoneNotificationTemplateAPI interface backed by the go-auth0 v2 SDK (m.Branding.Phone.Templates)
• Implements phoneNotificationTemplateResourceFetcher that lists phone notification templates and generates Terraform import blocks
• Extends parseResourceFetchers to accept *auth0.APIV2 alongside the existing v1 API, enabling v2 SDK usage in the terraform generation flow
• Adds auth0_phone_notification_template to the default resources list so it is included in generation by default

📚 References

• Closes Support auth0_phone_notification_template resources for tf generate #1512
• API Ref: https://auth0.com/docs/api/management/v2/branding/get-phone-templates

🔬 Testing

• 3 unit tests added for phoneNotificationTemplateResourceFetcher:
  • Success path with multiple templates
  • Empty list returns nil
  • API error propagation
• Existing TestTerraformInputs_ParseResourceFetchers updated and extended with a new test case for the resource
• All existing terraform tests continue to pass

go test ./internal/cli/ -run "Test.*[Tt]erraform" -v -count=1
go test ./internal/auth0/... -count=1

• Manually verified generation:

./auth0 terraform generate -o tf-import-test -r auth0_phone_notification_template                                                         3s
Fetching data from Auth0... done
 ▸    Output directory "tf-import-test" is not empty. Proceeding will overwrite the auth0_main.tf, auth0_import.tf and auth0_generated.tf files.
 Are you sure you want to proceed? Yes
Generating Terraform configuration... done
 ▸    Terraform resource config files generated successfully in: tf-import-test
 ▸    Review the config and generate the terraform state by running: 

        cd tf-import-test && ./terraform apply

 ▸    Once Terraform files are auto-generated, the terraform binary and auth0_import.tf files can be deleted.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)