[Build] `huggingface_hub.templates` absent from setuptools `packages` configuration

### Describe the bug

Building the package with setuptools produces a warning that `huggingface_hub.templates` is absent from the `packages` configuration:

```
Package 'huggingface_hub.templates' is absent from the `packages` configuration.
Python recognizes 'huggingface_hub.templates' as an importable package,
but it is absent from setuptools' `packages` configuration.
```

### Root cause

`src/huggingface_hub/templates/` is a directory containing `datasetcard_template.md` and `modelcard_template.md` but no `__init__.py`. `find_packages("src")` in `setup.py:114` only discovers packages with `__init__.py`, so `templates` is excluded. However, Python treats any directory as an importable namespace package, triggering the setuptools warning.

### Reproduction

```bash
python -m build --wheel --no-isolation
```

### Suggested fix

Either:
1. Switch to `find_namespace_packages("src")` which discovers namespace packages (directories without `__init__.py`), or
2. Add an empty `__init__.py` to `src/huggingface_hub/templates/`

Option 1 is the modern setuptools recommendation for projects that ship data directories alongside Python packages.

### Environment

- `huggingface_hub` main branch (commit `1e41293`)
- setuptools 80.x, Python 3.14

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