fix: restore correct assigned countries for virtual tyndp nodes

[daniel-rdt]

Closes #765.

Changes proposed in this Pull Request

This PR introduces a minor bug fix that wrongly assigned the country for the ITCO virtual node to FR instead of IT as manually set in build_tyndp_network.

The PR restores the declared country of TYNDP virtual nodes in base_network. As their coordinates are borrowed from a bus in a geographically different country, the coordinate-based assignment in _set_countries_and_substations is incorrectly reassigning them. buses holds the countries declared upstream in build_tyndp_network, before that reassignment and can be used to restore the correct value.

Note: The help of generative AI was used to generate docstrings for the introduced functions

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Checklist

Required:

• Security scans show no high-severity bugs, critical vulnerabilities, or exposed secrets.
• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.md.
• The description is human-written and any AI-generated content is marked.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• Changes in configuration options are added to config/test/*.yaml.
• Multiple climate years test passes locally (pixi run -e open-tyndp tyndp-cyears-test).
• For new data sources or versions, these instructions have been followed.
• Open-TYNDP SPDX license header is added to all touched files.
• Module docstrings are added to new Python scripts.
• New rules are documented in the appropriate doc/*.md files.
• Major features are documented in doc/index.md.

[tgilon]

Thank you @daniel-rdt for the fix. I think we could generalise a bit more to effectively link the two scripts via the global variable. Currently, the link is probably too weak to be robust.

[tgilon]

I understand the need to link the two files. You could create a dictionary instead and use that when adding the virtual nodes. This would be more robust.

[daniel-rdt]

Thanks @tgilon I refactored and generalised the addition of the virtual nodes in build_tyndp_network using a helper function and turning the global variable into a mapping dict.

Currently CI is failing due to the collect vs combine bug fixed in #797

[tgilon]

Thank you @daniel-rdt ! I think that this is more consistent internally. I have one final minor comment for the docstring. Otherwise, you could base this PR on #797 to solve the CI issue.

[tgilon]

The default is None, which implies the use of target_df

[daniel-rdt]

jep, good point. I clarified it.

[tgilon]

#797 has now been merged on master