Qiskit / qiskit-ibm-runtime / 29416747382

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Add optional `persist` argument to `FakeBackendV2.refresh()` method (#2977)

### Summary

`FakeBackendV2.refresh()` writes the updated `conf_*.json` /
`props_*.json` back into the package's own directory. When the package
is installed system-wide that directory is read-only
(`PermissionError`), and even when writable it's bad practice to mutate
files inside site-packages.

~Instead, let's keep the .json files in the `~/.qiskit` directory which
is already used for storing the IBM Quantum platform credentials.~

As suggested
[here](https://github.com/Qiskit/qiskit-ibm-runtime/issues/2976#issuecomment-4852615378),
the new proposal leaves the default behavior unchanged, i.e.,
`refresh()` tries to modify the backend json files in site-packages.
With ~`use_temp_dir=True`~ `persist=False`, the updated json files are
stored in a temp directory. This allows to use this method in
installations where the package's files are read-only or the user does
not have permissions to modify those files.

### Details and comments

- I tested this change locally and in the Arch Linux package and I
didn't have any issues.
- I used `os.path` instead of `pathlib.Path` because this was used
elsewhere. If you want I can refactor this using `pathlib`

Fixes #2976

- [ ] I didn't use LLM tooling, or only used it privately.
- [ ] I used the following tool to help write this PR description:
- [x] I used the following tool to generate or modify code: Claude Opus
4.8 for writing the unit tests

---------

Signed-off-by: Iyán Méndez Veiga <me@iyanmv.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Diego M. Rodríguez <diego@moreda.io>

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