Validate that lines: types defines the 380 kV level

[louisschlegel]

Closes #2166

Changes proposed in this Pull Request

Adds a field validator on lines: types so the config fails early with a clear message when no line type is defined for the 380 kV level, instead of the workflow failing later (e.g. in simplify_network when lines are mapped to 380 kV).

Tests added in test/test_config_schema.py for the default config and for a config without the 380 kV entry (pytest test/test_config_schema.py: 32 passed, 1 skipped).

Checklist

Required:

• 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.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.md files.