### Body

We had an issue where extra link class (`AIPlatformConsoleLink`) was removed in [PR](https://github.com/apache/airflow/pull/26836) without removing the class from the `provider.yaml` extra links this resulted in web server exception as shown in https://github.com/apache/airflow/pull/28449


**The Task:**
Add validation that classes of extra-links in provider.yaml are importable

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
print("Detect unregistered integrations")
    all_integration_names = set(get_all_integration_names(yaml_files))
    for (yaml_file_path, provider_data), resource_type in product(
        yaml_files.items(), ["sensors", "operators", "hooks"]
    ):
        resource_data = provider_data.get(resource_type, [])
        current_names = {r["integration-name"] for r in resource_data}
        invalid_names = current_names - all_integration_names
        if invalid_names:
            errors.append(
                f"Incorrect content of key '{resource_type}/integration-name' in file: {yaml_file_path}. "
                f"Invalid values: {invalid_names}"
            )
    for (yaml_file_path, provider_data), key in product(
        yaml_files.items(), ["source-integration-name", "target-integration-name"]
    ):
        resource_data = provider_data.get("transfers", [])
        current_names = {r[key] for r in resource_data}
        invalid_names = current_names - all_integration_names
        if invalid_names:
            errors.append(
                f"Incorrect content of key 'transfers/{key}' in file: {yaml_file_path}. "
                f"Invalid values: {invalid_names}"
            )
def check_doc_files(yaml_files: dict[str, dict]):