[master378] Send the filtered retain trailing event with a client specific Retain = false

[romanett]

Description

Backport of #4453 to the 1.5.378 line.

When SupportsFilteredRetain is true and a condition drops out of a client's where clause, the server sends one trailing event to that client. Before this change the trailing event carried the condition's real Retain value, typically true, so a spec-following client kept an alarm on its display that it should have dropped. OPC 10000-9, 5.5.2 (Figure 11) and the "Retain sent" column of Table B.3 require that event to be sent with a client specific Retain = false.

The value is per client and per filter, so it is applied inside the monitored item:

• CanSendFilteredAlarm gains an out bool overrideRetain parameter that is true only for the trailing out-of-scope event. The protected signature is changed in place rather than overloaded.
• For that event the event fields are read through a new internal FilteredRetainTarget decorator. It delegates everything to the original IFilterTarget and returns false for a Retain value clause the target actually resolves. A clause the type check rejects stays null so the field list keeps its shape.
• The shared InstanceStateSnapshot that ReportEvent fans out to every monitored item is never touched, so other subscriptions whose filter the condition still passes keep receiving the server's real value. The queued EventFieldList keeps the original instance as its Handle, so reference-based duplicate detection and node manager handle lookups are unaffected.

Differences from the master change: this branch has neither the #4377 filtered-retain rework nor docs/AlarmsAndConditions.md, so the decorator is written against the object-based IFilterTarget of this line, the existing tracking logic is left as it is, and there is no documentation change. The reflection-based test helper reads the new out parameter back from the argument array.

Tests added in FilterRetainTests:

• the step-5 event from the issue queued through two monitored items sharing one snapshot: the out-of-scope client receives Retain = false, the still-passing client receives true, and the snapshot itself still reports true;
• the override applies to the single trailing event only and the server value returns once the condition re-enters scope;
• an unresolvable Retain clause stays null;
• the existing 16-step Table B.3 test now also asserts the "Retain sent" column for the trailing rows.

Related Issues

• Fixes SupportsFilteredRetain: trailing out-of-scope event is sent with the server's Retain value instead of the client-specific Retain = false #4449 for 1.5.378
• Backport of Send the filtered retain trailing event with a client specific Retain = false #4453

Checklist

• I have signed the CLA and read the CONTRIBUTING doc.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added all necessary documentation.
• I have verified that my changes do not introduce (new) build or analyzer warnings.
• I ran all tests locally using the UA.slnx solution against at least .net framework and .net 10, and all passed.
• I fixed all failing and flaky tests in the CI pipelines and all CodeQL warnings.
• I have addressed all PR feedback received.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.35%. Comparing base (0744066) to head (786ac39).

Files with missing lines	Patch %	Lines
...Subscription/MonitoredItem/FilteredRetainTarget.cs	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           master378    #4454      +/-   ##
=============================================
- Coverage      60.35%   60.35%   -0.01%     
=============================================
  Files            378      379       +1     
  Lines          79155    79181      +26     
  Branches       13850    13853       +3     
=============================================
+ Hits           47777    47788      +11     
- Misses         26961    26976      +15     
  Partials        4417     4417              

Files with missing lines	Coverage Δ
...Server/Subscription/MonitoredItem/MonitoredItem.cs	64.42% <100.00%> (+1.36%)	⬆️
...Subscription/MonitoredItem/FilteredRetainTarget.cs	88.88% <88.88%> (ø)

... and 8 files with indirect coverage changes

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