Fix ServerTimestamp update on read of stored/static nodes (#4257)

[romanett]

Summary

Backports the fix for ServerTimestamp updating on Read operations for stored/static nodes to master378 (1.5.378), addressing issue #4257 (part 2).

Details

• In CustomNodeManager and CoreNodeManager, ensure ServerTimestamp is stamped with the read time for stored/static nodes whose SourceTimestamp is older than
  eadTime (e.g. after a Write).
• Retain matching SourceTimestamp and ServerTimestamp for dynamically produced values generated during Read (such as ServerStatus children).
• Adds unit test ReadStaticVariableStampsFreshServerTimestampAfterWriteAsync in ReferenceServerTest.cs to verify that after writing to a static variable, subsequent reads return ServerTimestamp stamped with the current read time.

Fixes #4257 (ServerTimestamp behavior)

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.21%. Comparing base (db256c6) to head (9aab965).

Files with missing lines	Patch %	Lines
...aries/Opc.Ua.Server/NodeManager/CoreNodeManager.cs	0.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (62.50%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@              Coverage Diff              @@
##           master378    #4260      +/-   ##
=============================================
+ Coverage      60.19%   60.21%   +0.01%     
=============================================
  Files            378      378              
  Lines          79067    79069       +2     
  Branches       13838    13840       +2     
=============================================
+ Hits           47598    47614      +16     
+ Misses         27047    27034      -13     
+ Partials        4422     4421       -1     

Files with missing lines	Coverage Δ
...ies/Opc.Ua.Server/NodeManager/CustomNodeManager.cs	60.92% <100.00%> (-0.20%)	⬇️
...aries/Opc.Ua.Server/NodeManager/CoreNodeManager.cs	13.42% <0.00%> (-0.01%)	⬇️

... and 6 files with indirect coverage changes

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