Add DiffSL export functionality

[martinjrobins]

Description

This PR adds the ability to export PyBaMM models to the DiffSL format, enabling models to be used with the DiffSL solver ecosystem.

Note this was started before the move from develop to main, in the process some commits got added, but these don't affect the diff

Changes

• New DiffSLExport class in src/pybamm/expression_tree/operations/diffsl.py:

  • Converts PyBaMM models to DiffSL format

• Unit and integration tests:

  • Added tests for ODE export (test_ode)
  • Added tests for heat equation export (test_heat_equation)
  • Integration tests for SPM, SPMe, and DFN models

Related Issues

Closes #5262

Checklist

• Tests added
• Documentation/docstrings updated (inline in code)
• CHANGELOG updated
• Code follows style guidelines

[Copilot]

Pull request overview

This PR adds the ability to export discretized PyBaMM models to the DiffSL format, enabling integration with the DiffSL solver ecosystem (specifically pydiffsol). The implementation converts PyBaMM expression trees to DiffSL code that can be compiled and solved using diffsol's ODE/DAE solvers.

Changes:

• New DiffSLExport class for converting PyBaMM models to DiffSL format
• Unit tests covering basic ODE and heat equation (PDE) exports
• Integration tests validating exports against actual pydiffsol solver for SPM, SPMe, and DFN battery models

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
src/pybamm/expression_tree/operations/diffsl.py	Core implementation of DiffSL export functionality with methods to convert PyBaMM symbols, vectors, and matrices to DiffSL format
tests/unit/test_expression_tree/test_operations/test_diffsl_export.py	Unit tests for basic ODE and PDE model exports
tests/integration/test_solvers/test_diffsl_export.py	Integration tests validating DiffSL export against pydiffsol solver for multiple battery models
src/pybamm/__init__.py	Export of DiffSLExport class to public API
pyproject.toml	Addition of pydiffsol as dev dependency for testing
CHANGELOG.md	Documentation of new feature

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.05%. Comparing base (c9121be) to head (0ddf5e0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5370      +/-   ##
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+ Coverage   98.03%   98.05%   +0.02%     
==========================================
  Files         327      328       +1     
  Lines       28690    29050     +360     
==========================================
+ Hits        28126    28486     +360     
  Misses        564      564              

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