mpi: Localise SubDimension thicknesses covering a whole rank exactly

[ggorman]

index_glb_to_loc(offset, side) returned one past the last local index when the offset covered the whole local subdomain, so a rank lying entirely in a left/right boundary layer got a thickness one larger than its extent and the loop ran past its data: with 4 ranks x 16 points and a 20-point layer, rank 0 and rank 3 got 17, corrupting the heap.

The function now always returns an index, as asked on #2993, and the callers convert it to a count (left, right and middle alike; middle was already correct on main). #3034's call-site clamp also fixes the left/right symptom; this is the alternative that keeps the function's contract, and #3034's tests pass on top of it without the clamp.

Tests: a serial index case in test_data.py; test_mpi.py::TestOperatorAdvanced::test_thickness_covering_ranks[left|right|middle] (4 ranks). With the whole fix reverted the three MPI cases fail; with only the conversion reverted, middle fails (15 != 16).

Refs #2993, #3034.

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[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.21%. Comparing base (2707ac9) to head (22fdd71).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_mpi.py	13.04%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3068      +/-   ##
==========================================
- Coverage   84.23%   84.21%   -0.02%     
==========================================
  Files         258      258              
  Lines       57070    57091      +21     
  Branches     4874     4874              
==========================================
+ Hits        48072    48079       +7     
- Misses       8170     8188      +18     
+ Partials      828      824       -4     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.51% <60.00%> (+<0.01%)	⬆️
pytest-gpu-gcc-	79.00% <33.33%> (-0.01%)	⬇️
pytest-gpu-icx-	78.90% <33.33%> (-0.02%)	⬇️
pytest-gpu-nvc-nvidiaX	69.08% <60.00%> (-0.01%)	⬇️

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