[http-client-python] Remove setuptools warnings for pygen template directories

## Description

Building `@typespec/http-client-python` with current setuptools reports warnings that the data-only template directories are absent from the package configuration:

```text
Package 'pygen.codegen.templates' is absent from the `packages` configuration.
Package 'pygen.codegen.templates.packaging_templates' is absent from the `packages` configuration.
```

`generator/setup.py` uses `find_packages()` with `include_package_data=True`, while `generator/MANIFEST.in` recursively includes the Jinja templates. Since the template directories do not contain `__init__.py`, `find_packages()` does not discover them, but recent setuptools versions treat them as importable packages and warn about the ambiguous configuration.

## Reproduction

From `packages/http-client-python`:

```shell
npm run build
```

The warnings are emitted while building the pygen wheel.

## Current behavior

The wheel currently appears complete: `pygen-0.1.0-py3-none-any.whl` contains all 44 `.jinja2` template files, including the seven files under `pygen/codegen/templates/packaging_templates`. This is therefore a packaging-configuration warning rather than a known missing-artifact bug.

## Expected behavior

The pygen package configuration should explicitly and unambiguously include its template data, and building the wheel should not emit the missing-package warnings.

## Acceptance criteria

- Building the pygen wheel does not report that `pygen.codegen.templates` or `pygen.codegen.templates.packaging_templates` is absent from the package configuration.
- All existing Jinja templates remain present in the built wheel.
- `jinja2.PackageLoader("pygen.codegen", "templates")` and the packaging-template loader continue to work from an installed wheel.
- Add or update a packaging test that verifies the required template files are included in the wheel.