avoid Jacobian resets for endpoint checkpoints

[martinjrobins]

fixes #223

Motivation

• Avoid redundant internal Jacobian reinitialisations when taking the first and last checkpoints during checkpointed solves to reduce unnecessary work and potential side-effects.
• Provide a way to clone solver state without triggering Jacobian updates so forward solver clones used for checkpointing are side-effect free.

Description

• create the trait method state_clone on OdeSolverMethod.
• Use state_clone for the initial and final checkpoints in solve_with_checkpointing and solve_dense_with_checkpointing instead of taking a full checkpoint that forces Jacobian updates.
• Implement state_clone for ExplicitRk, Sdirk, and Bdf by returning a plain clone of their internal state.
• Stop the eager Jacobian reset during Bdf Clone so cloning does not trigger an immediate Jacobian recalculation.

Testing

• No automated tests were run for this change.

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.24%. Comparing base (df915d6) to head (41fd84e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   74.22%   74.24%   +0.02%     
==========================================
  Files          85       85              
  Lines        9377     9382       +5     
==========================================
+ Hits         6960     6966       +6     
+ Misses       2417     2416       -1     

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