Fix FiniteVolume2D boundary extrapolation on meshes with fewer than 3 nodes

[aabills]

Description

FiniteVolume2D.boundary_value_or_flux read the second and second-last node spacings in both directions (d_nodes_lr[1], d_nodes_lr[-2], d_nodes_tb[1], d_nodes_tb[-2]) before branching on side and extrapolation order. Any 2D submesh with fewer than 3 nodes in either direction therefore raised IndexError: index 1 is out of bounds for axis 0 with size 1 for every boundary operator, including a plain BoundaryValue(var, "right") or BoundaryIntegral(var, "right") that only extrapolates in lr.

Reproduce on main: pybamm.lithium_ion.BasicDFN2D() with its z_2d var_pts set to 2, solved via pybamm.Simulation.

Spacings are now looked up lazily through a local node_spacing(direction, index) helper, called only by the branch that uses them. When the mesh has too few nodes for the requested extrapolation, it raises a pybamm.DiscretisationError naming the order, variable, direction, and required vs. available node count.

Meshes with at least 3 nodes in both directions use the same spacings as before, so their results are unchanged.

Not covered here: with z_2d set to 1, the model gets past boundary extrapolation and then fails in harmonic_mean, which is a separate bug.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Tests

• New TestExtrapolationCoarseMesh2D in tests/unit/test_spatial_methods/test_finite_volume_2d/test_extrapolation.py: with 2 tb nodes, linear and quadratic left/right values and the right boundary integral are exact; six too-coarse combinations raise DiscretisationError. All 8 fail on main.
• 2D / finite-volume unit subset: 468 passed. Pre-commit clean.

🤖 Generated with Claude Code

[github-actions]

Bencher Report

Project	PyBaMM
Branch	fix/fv2d-coarse-mesh-boundary-value
Testbed	bare-metal

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.09%. Comparing base (445ba33) to head (99fec58).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5839   +/-   ##
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  Coverage   98.09%   98.09%           
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  Files         346      346           
  Lines       35354    35355    +1     
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+ Hits        34680    34681    +1     
  Misses        674      674           

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