Improve composite model performance and sparsity

[MarcBerliner]

Description

Improved performance of the composite models by removing dense blocks in the Jacobian. Also adds a jacobian sparsity helper in IDAKLUSolver

Benchmark: composite SPMe (particle_phases=(2, 1)), 1C discharge 3600s

Metric	main	This PR	Change
surface form = algebraic
System size	124x124	127x127	+3 algebraic equations
Jacobian nnz	3,129	866	-72%
Sparsity	79.7%	94.6%
Median solve time	10.7 ms	6.6 ms	-38%
Max voltage diff	--	3.78e-12 V
surface form = differential
System size	124x124	127x127	+3 algebraic equations
Jacobian nnz	3,129	866	-72%
Sparsity	79.7%	94.6%
Median solve time	12.6 ms	9.7 ms	-23%
Max voltage diff	--	1.70e-12 V

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.30%. Comparing base (9209226) to head (02bedc6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pybamm/solvers/idaklu_solver.py	88.88%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5439      +/-   ##
==========================================
- Coverage   98.33%   98.30%   -0.04%     
==========================================
  Files         336      337       +1     
  Lines       30699    31025     +326     
==========================================
+ Hits        30189    30498     +309     
- Misses        510      527      +17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.