Fix experiment initial guess regression

[MarcBerliner]

Description

For experiment steps after the initial run, always get initial guesses from the previous model's state trajectory instead of the current model's initial conditions. Raise if the state variable cannot be found from the previous model

Fixes #5449

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.28%. Comparing base (37dfaa5) to head (7b2d567).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5452   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files         337      337           
  Lines       30930    30933    +3     
=======================================
+ Hits        30401    30404    +3     
  Misses        529      529           

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