Unit overhaul 2/6: Centralize Q-index validation via verify_Q_index

[henrikjacobsenfys]

Replace the AnalysisBase._verify_Q_index method and the various inline Q_index type/bounds checks scattered across the model hierarchy with the shared verify_Q_index helper (added in the previous PR). Callers now delegate validation to a single function with consistent error messages and behavior (TypeError for non-int, IndexError for negative/out-of-range, upper-bound deferred when Q is None).

Migrated: Analysis, Analysis1d, AnalysisBase, Experiment, ModelBase, InstrumentModel, DiffusionModelBase and DeltaLorentz. Tests updated to the unified messages.

Net: 15 files, +71/−236 (removes duplicated validation blocks). All unit tests pass.

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Stack — splitting the monolithic "Unit system overhaul" (#217) into a reviewable series. Merge bottom-up:

1. Add unit-conversion utilities and FitTarget → develop (Unit overhaul 1/6: Add unit-conversion utilities and FitTarget #221)
2. Centralize Q-index validation via verify_Q_index → split/01-unit-utils (Unit overhaul 2/6: Centralize Q-index validation via verify_Q_index #222) (this PR)
3. Store Q as a scipp Variable in 1/angstrom → split/02a-verify-q-index (Unit overhaul 3/6: Store Q as a scipp Variable in 1/angstrom #223)
4. Separate model unit into x_unit/y_unit + update tutorials → split/02b-q-variable (Unit overhaul 4/6: Separate unit into x_unit/y_unit and update tutorials #224)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.01%. Comparing base (36bef4d) to head (b170048).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #222      +/-   ##
===========================================
+ Coverage    97.92%   98.01%   +0.09%     
===========================================
  Files           53       53              
  Lines         3707     3685      -22     
  Branches       660      640      -20     
===========================================
- Hits          3630     3612      -18     
+ Misses          45       43       -2     
+ Partials        32       30       -2     

Flag	Coverage Δ
unittests	98.01% <100.00%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easydynamics/analysis/analysis.py	95.06% <100.00%> (+0.02%)	⬆️
src/easydynamics/analysis/analysis1d.py	98.60% <100.00%> (+0.01%)	⬆️
src/easydynamics/analysis/analysis_base.py	100.00% <ø> (ø)
src/easydynamics/experiment/experiment.py	97.66% <100.00%> (ø)
...mics/sample_model/diffusion_model/delta_lorentz.py	97.57% <100.00%> (-0.01%)	⬇️
...mple_model/diffusion_model/diffusion_model_base.py	96.87% <100.00%> (+2.93%)	⬆️
src/easydynamics/sample_model/instrument_model.py	100.00% <100.00%> (ø)
src/easydynamics/sample_model/model_base.py	97.56% <100.00%> (-0.10%)	⬇️

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