Raise clear error for PEtab YAML with empty 'problems' section

[dweindl]

Fixes the remaining part of #218.

The originally reported KeyError: 'parameter_file' no longer occurs, since Problem.from_yaml now builds a pydantic ProblemConfig, which already gives parameter_file/sbml_files sane defaults. However, the same class of problem still exists for problems: ProblemConfig.problems defaults to [] when the key is missing from the YAML (or is given explicitly as problems: []), and Problem.from_yaml then indexes into it unconditionally (config.problems[0]), raising a raw IndexError (is_composite_problem() similarly raised a raw KeyError beforehand).

So this isn't really about a "missing" key — it's about an empty problems section, however it comes to be empty. This adds a clear ValueError for that case, and a parametrized regression test covering both the missing-key and explicit-empty-list cases.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.15%. Comparing base (02fd94e) to head (764b2be).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #521   +/-   ##
=======================================
  Coverage   76.14%   76.15%           
=======================================
  Files          67       67           
  Lines        7526     7528    +2     
  Branches     1341     1342    +1     
=======================================
+ Hits         5731     5733    +2     
  Misses       1294     1294           
  Partials      501      501           

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