fix: resolve constant-constant constraint crashes and document H2 limits config

[omari91]

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Changes proposed in this Pull Request

• Fixes solver crashes due to constant constraints: Replaced direct n.model.add_constraints(...) calls with a new safe_add_constraint(...) wrapper function in scripts/pypsa-de/additional_functionality.py. This safely bypasses adding inequality constraints where both the left-hand side and right-hand side resolve to 0 (which previously caused solvers like Highs/Gurobi to crash with ValueError: Both sides of the constraint are constant).
• Refactored constraint logic: Applied safe_add_constraint systematically across functions such as h2_import_limits, h2_production_limits, capacity_limits, limit_co2, and auxiliary_flow_limits.
• Cleaned up redundant code: Removed an obsolete duplicate code block within h2_import_limits() that manually checked for empty links, as the new wrapper handles this automatically.
• Improved configuration visibility: Explicitly documented the pypsa-de.h2_import_limits.enable configuration toggle in config.de.yaml.
• Updated CHANGELOG.md to reflect the changes.

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A brief description of the changes has been added to Changelog.md.
  If applicable:
• Changes in configuration options are reflected in scripts/lib/validation. (N/A: no pypsa-de schemas were found)
• For new data sources or versions, these instructions have been followed. (N/A)
• New rules are documented in the appropriate doc/*.rst files. (N/A)