Create `airflow.core.*` module structure for Server-client separation

Move the current server components under `airflow.core.*` module structure & namespace to establish clear boundaries for AIP-72 server-client separation to clearly mark server components. Example: - `airflow.models.*` → `airflow.core.models.*` - `airflow.jobs.*` → `airflow.core.jobs.*` - `airflow.executors.*` → `airflow.core.executors.*` - `airflow.api_fastapi.*` → `airflow.core.api.*` ## Success Criteria - All server components in `airflow.core.*` namespace - No internal code imports from old locations - All tests pass with new structure - Clear separation between server and shared components