Composite electrode bug-fix for swelling submodel (#5272)

Author: BradyPlanden

CHANGELOG.md

@@ -7,6 +7,7 @@
 ## Bug fixes
 
 - Fixed a bug where simulations whose initial conditions violated the events could continue solving. ([#5260](https://github.com/pybamm-team/PyBaMM/pull/5260))
+- Fixed an issues with composite electrode and "swelling only" mechanics sub-models, which were not creating the cell thickness variable. ([#5272](https://github.com/pybamm-team/PyBaMM/pull/5272))
 
 # [v25.10.0](https://github.com/pybamm-team/PyBaMM/tree/v25.10.0) - 2025-10-29