Fix FiniteDifferences.replace_params losing accuracy

[astanziola]

Summary

• OnGrid.replace_params() was inherited by FiniteDifferences but only passed (new_params, domain), dropping the accuracy static field (silently reverting to default 8)
• This caused lax.scan pytree metadata mismatches when users created FD fields with non-default accuracy
• Override replace_params in FiniteDifferences to preserve accuracy

Fixes the jaxdf side of ucl-bug/jwave#224.

Added parametrized unit tests and a lax.scan integration test to prevent regression.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (5150d6d) to head (fb54866).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   93.43%   93.45%   +0.01%     
==========================================
  Files          15       15              
  Lines         945      947       +2     
==========================================
+ Hits          883      885       +2     
  Misses         62       62              

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