fix(fracmanager): do not write state on fracmanager init

[eguguchkin]

Before: the storage-state.json file was created during the initialization of the FracManager structure.
Now: The file is written only when the storage state is explicitly changed.

This allows us to avoid creating a temporary directory and cleaning it up in tests that create a FracManager.

---

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.86%. Comparing base (36016e6) to head (810fef6).

Files with missing lines	Patch %	Lines
fracmanager/storage_state.go	63.63%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
- Coverage   71.88%   71.86%   -0.02%     
==========================================
  Files         199      199              
  Lines       14570    14570              
==========================================
- Hits        10473    10471       -2     
- Misses       3364     3365       +1     
- Partials      733      734       +1     

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