fix: raise typed exceptions instead of asserts in modifier validation

[henryiii]

🤖 AI text below 🤖

Part of the code review in #2706.

Summary

• Replace assert self.interpcode in [...] in histosys_combined.__init__ and normsys_combined.__init__ with raise InvalidInterpCode(msg) so the check is never silently dropped under python -O
• Replace assert (mask_this_sample == masks[modname]).all() in staterror_builder.finalize with raise InvalidModel(msg) naming the offending modifier, guarding the real invariant that shared staterror parameters must have identical bin masks across samples
• Add 8 new parametrized tests in tests/test_modifiers.py covering invalid interpcode values for both histosys and normsys

Test plan

• pytest tests/test_modifiers.py tests/test_pdf.py tests/test_interpolate.py — 144 passed, 6 skipped
• prek -a --quiet (ruff + pre-commit) — clean
• test_histosys_invalid_interpcode parametrized over ["code1", "code4", "code3", "bad"] all raise InvalidInterpCode
• test_normsys_invalid_interpcode parametrized over ["code0", "code2", "code4p", "bad"] all raise InvalidInterpCode

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 7db1c19e-ab7a-4444-ae80-174c6352e2c5

📥 Commits

Reviewing files that changed from the base of the PR and between 0e54a51 and 021236a.

📒 Files selected for processing (4)

• src/pyhf/modifiers/histosys.py
• src/pyhf/modifiers/normsys.py
• src/pyhf/modifiers/staterror.py
• tests/test_modifiers.py

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[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.38%. Comparing base (0e54a51) to head (021236a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pyhf/modifiers/staterror.py	20.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2714      +/-   ##
==========================================
- Coverage   98.28%   96.38%   -1.90%     
==========================================
  Files          65       65              
  Lines        4305     4313       +8     
  Branches      465      468       +3     
==========================================
- Hits         4231     4157      -74     
- Misses         46      122      +76     
- Partials       28       34       +6     

Flag	Coverage Δ
contrib	?
doctest	?
unittests-3.10	?
unittests-3.11	96.38% <69.23%> (-0.09%)	⬇️
unittests-3.12	?
unittests-3.13	?
unittests-3.9	?

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