Fix DEModel._expr_is_time_dependent not accounting for event assignment targets

[dweindl]

Fixes #3046.

Relax the faulty check. Better to track more root functions than missing some. Will optimize after #3031.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.45%. Comparing base (baed9cd) to head (88053e8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
python/sdist/amici/de_model.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3047      +/-   ##
==========================================
- Coverage   78.10%   77.45%   -0.65%     
==========================================
  Files         311      311              
  Lines       20580    20582       +2     
  Branches     1499     1499              
==========================================
- Hits        16073    15941     -132     
- Misses       4497     4631     +134     
  Partials       10       10              

Flag	Coverage Δ
cpp	70.71% <85.71%> (-0.01%)	⬇️
cpp_python	37.18% <71.42%> (-0.01%)	⬇️
petab	45.83% <71.42%> (-0.03%)	⬇️
petab_sciml	13.75% <0.00%> (-0.01%)	⬇️
python	69.37% <85.71%> (-0.01%)	⬇️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
python/sdist/amici/de_model.py	91.53% <85.71%> (-0.12%)	⬇️

... and 5 files with indirect coverage changes

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