fix(pypsa_io): include line shunt susceptance in per-grid to_pypsa export

[nader-00]

Description

The mv/lv per-grid export path dropped the "b" column from lines_df while the full-grid path (mode=None) kept it, so PyPSA silently defaulted it to 0 instead of the real value on the per-grid path. Align both paths to select the same Line columns, and add a regression test that injects a distinguishing non-zero b to catch the divergence (issue #654).

No .rename(columns={"r_pu": "r", "x_pu": "x"}) belong to Line, only to Transformers, so it was kept untouched.

Fixes #654

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them the setup.py, and if needed the rtd_requirements.txt, the eDisGo_env.yml and the eDisGo_env_dev.yml.
• I have added new features to the corresponding whatsnew file