Refactor: replace public-facing asserts with explicit exceptions

[arafune]

Convert public-facing assert checks to explicit TypeError/ValueError for consistent API behavior. Tests updated; full suite passes locally.

[codecov]

Codecov Report

❌ Patch coverage is 16.62651% with 692 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.70%. Comparing base (c220235) to head (021259c).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/arpes/plotting/utils.py	0.00%	78 Missing and 3 partials ⚠️
src/arpes/xarray_extensions/accessor/general.py	3.44%	14 Missing and 14 partials ⚠️
src/arpes/analysis/derivative.py	0.00%	12 Missing and 12 partials ⚠️
src/arpes/analysis/sarpes.py	0.00%	22 Missing ⚠️
src/arpes/xarray_extensions/accessor/property.py	0.00%	10 Missing and 10 partials ⚠️
src/arpes/models/band.py	0.00%	16 Missing and 3 partials ⚠️
src/arpes/analysis/forward_conversion.py	0.00%	12 Missing and 6 partials ⚠️
src/arpes/utilities/selections.py	5.26%	9 Missing and 9 partials ⚠️
src/arpes/io.py	5.88%	12 Missing and 4 partials ⚠️
src/arpes/utilities/conversion/kx_ky_conversion.py	5.88%	8 Missing and 8 partials ⚠️
... and 78 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
- Coverage   72.30%   69.70%   -2.61%     
==========================================
  Files         228      228              
  Lines       14640    15037     +397     
  Branches     1222     1572     +350     
==========================================
- Hits        10585    10481     -104     
- Misses       3825     4151     +326     
- Partials      230      405     +175     

Flag	Coverage Δ
unittests	69.70% <16.62%> (-2.61%)	⬇️

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