Fix PotentialPair1plus1D/2plus1D normalisation for parallel electrodes (#5467)

[kamal20122012]

Summary

• The 1+1D and 2+1D PotentialPair current-collector submodels apply a Neumann BC at the positive tab of a single modelled electrode pair, but the boundary condition was computed from the total cell current (applied_current_density * A_cc) rather than the per-electrode-pair current. Because A_cc already includes n_electrodes_parallel, the gradient of phi_s_cp at the tab was overstated by that factor whenever more than one electrode pair was connected in parallel — voltage drops in the current collector were inflated, the lower voltage cutoff was hit prematurely, and the reported discharge capacity was approximately 1 / n_electrodes_parallel of the correct value.
• Fix: divide by n_electrodes_parallel so the BC uses the current per modelled electrode pair. No effect for the default n=1 case.
• Added an integration regression test that compares discharge capacity between an equivalent wide single-electrode configuration and a two-parallel-electrode configuration with the same total current-collector area; capacities now match.

Fixes #5467

Test plan

• Reproduction script from the issue: parallel config now reports 40.84 A·h (matches wide single-electrode case) instead of 22.36 A·h.
• pytest tests/integration/test_models/test_full_battery_models/test_lithium_ion/test_dfn.py::TestDFN::test_potential_pair_1plus1D_parallel_electrodes (passes; verified to fail on pre-fix code)
• pytest tests/unit/test_models/test_full_battery_models/test_lithium_ion -k "1plus1D or 2plus1D or potential_pair" — 50 passed
• pre-commit run --files <changed files> — all hooks pass (ruff-check, ruff-format, blacken-docs, file-checks, sp-repo-review)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (19c2d57) to head (8db311f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5478   +/-   ##
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  Coverage   98.15%   98.15%           
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  Files         338      338           
  Lines       31115    31122    +7     
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+ Hits        30542    30549    +7     
  Misses        573      573           

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