Change save callbacks from PresetTimeCallback to FunctionCallingCallback

[albertomercurio]

Checklist

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• Please read Contributing to Quantum Toolbox in Julia.
• Any code changes were done in a way that does not break public API.
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Description

The current implementation of the save callbacks relies on PresetTimeCallback, which adds tstops to the integrator and can lead to extra, unnecessary evaluations of the differential equation. By contrast, using FunctionCallingCallback avoids adding tstops; it simply interpolates the solution at the desired time points.

This approach also resolves the issue where the solution’s stability depends on whether the e_ops argument is used.

We still need to wait this PR to be merged if we want type stability.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.20%. Comparing base (d48371c) to head (51225c4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ution/callback_helpers/mesolve_callback_helpers.jl	80.00%	1 Missing ⚠️
...ution/callback_helpers/sesolve_callback_helpers.jl	75.00%	1 Missing ⚠️
...tion/callback_helpers/smesolve_callback_helpers.jl	75.00%	1 Missing ⚠️
...tion/callback_helpers/ssesolve_callback_helpers.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #410   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files          46       46           
  Lines        2918     2918           
=======================================
  Hits         2749     2749           
  Misses        169      169           

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