feat: add PaddlePaddle scanner test coverage

[mldangelo]

Description

The PaddlePaddle scanner (`paddle_scanner.py`) handles `.pdmodel` and `.pdiparams` files but has limited test coverage. Add comprehensive tests including both benign and malicious model samples.

Background

PaddlePaddle (PaddlePaddle.org) is a popular ML framework, especially in China. Model files can contain serialized Python objects similar to pickle, making them a potential attack vector.

Suggested approach

1. Create minimal test fixtures (benign and malicious `.pdmodel` files)
2. Test detection of suspicious operations in PaddlePaddle models
3. Test graceful handling of corrupted/invalid files
4. Test scanner registration and priority in the registry

References

• `modelaudit/scanners/paddle_scanner.py` — scanner implementation
• `tests/scanners/` — existing scanner test patterns
• `docs/agents/new-scanner-quickstart.md` — scanner development guide

Getting started

```bash
uv sync --extra all-ci
uv run pytest tests/scanners/ -v -k "paddle"
```

Acceptance criteria

• At least 5 test cases covering benign, malicious, and edge cases
• Tests create minimal fixtures (no large binary files committed)
• All tests pass: `uv run pytest -n auto -m "not slow"`