Pass handle_missing_countries param to build_dh_areas rule

[louisschlegel]

Closes #2359

Changes proposed in this Pull Request

scripts/build_dh_areas.py reads snakemake.params["handle_missing_countries"], but the build_dh_areas rule had no params block. This adds it, reading sector: district_heating: dh_areas: handle_missing_countries from the config.

Tested by running build_dh_areas with the dh_areas dataset (341.5) and onshore regions including a country without DH data (AL). On master it fails with AttributeError: 'Params' object has no attribute 'handle_missing_countries'; with this change the rule completes and AL is filled.

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.