INTERNALERROR> AttributeError: 'Config' object has no attribute 'cache'

## Bug Report

**Environment**
`pytest==8.3.0`, `pytest-memray==1.7.0`

**Repro Steps**
`pytest --memray -p no:cacheprovider` gets the error
```sh
if _config.cache is not None:
AttributeError: 'Config' object has no attribute 'cache'
```

This is due to here: https://github.com/bloomberg/pytest-memray/blob/e2d0aed0d85b24e3ee45f413c3d4d19a9f904d2b/src/pytest_memray/marks.py#L206
```python
if _config.cache is not None:
cache = _config.cache.get(f"memray/{_test_id}", {})
previous = cache.get("total_allocated_memory", float("inf"))
```

**Expected behavior/code**
Able to run pytest memray without the cacheprovider plugin

**Possible Solution**
Replace ` if _config.cache is not None:` with `if getattr(_config, 'cache', None) is not None:`

Inspect src/pytest_memray/marks.py around line 206 and reproduce the failure with pytest --memray -p no:cacheprovider. Check Config behavior when the cacheprovider plugin is disabled. Done means the command runs without AttributeError while preserving memray behavior.

Written by the indexing model from the issue text.