Refactor client state persistence and add more tests

[marcschier]

Proposed changes

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Related Issues

• Fixes #

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• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

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• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
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[codecov]

Codecov Report

❌ Patch coverage is 73.78788% with 519 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.36%. Comparing base (8f5a042) to head (73cdc54).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...braries/Opc.Ua.Client/Session/SessionExtensions.cs	30.98%	95 Missing and 3 partials ⚠️
...s/Opc.Ua.Client/Session/SessionClientExtensions.cs	74.18%	85 Missing and 10 partials ⚠️
...raries/Opc.Ua.Client/NodeCache/NodeCacheContext.cs	85.15%	58 Missing and 25 partials ⚠️
Libraries/Opc.Ua.Client/Browser.cs	74.55%	64 Missing and 7 partials ⚠️
...raries/Opc.Ua.Client/Subscription/MonitoredItem.cs	53.27%	38 Missing and 12 partials ⚠️
...ies/Opc.Ua.Client/Session/DefaultSessionFactory.cs	76.06%	39 Missing and 6 partials ⚠️
...braries/Opc.Ua.Client/Subscription/Subscription.cs	81.92%	21 Missing and 9 partials ⚠️
Libraries/Opc.Ua.Client/NodeCache/NodeCache.cs	78.26%	9 Missing and 1 partial ⚠️
...ent.ComplexTypes/TypeResolver/NodeCacheResolver.cs	30.76%	9 Missing ⚠️
...s/Opc.Ua.Client/Subscription/MonitoredItemState.cs	46.15%	7 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3303      +/-   ##
==========================================
+ Coverage   59.21%   59.36%   +0.15%     
==========================================
  Files         320      323       +3     
  Lines       76270    75777     -493     
  Branches    13300    13329      +29     
==========================================
- Hits        45166    44988     -178     
+ Misses      27034    26723     -311     
+ Partials     4070     4066       -4     

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