Fix unordered server endpoint validation

[MrAlaskan]

Summary

This PR makes Session.ValidateServerEndpoints() compare ServerEndpoints and UserIdentityTokens as unordered sets and adds regression tests that verify OpenAsync() no longer rejects legal servers that return the same endpoint data in a different order.

Problem

According to OPC UA Part 4, the client validates the endpoint set returned by CreateSessionResponse.ServerEndpoints against the endpoint set observed during discovery. The specification describes these values as a set of endpoint descriptions filtered by the relevant transport profile, not as an ordered array that must preserve the exact discovery-time enumeration order.

Previously, Session.ValidateServerEndpoints() first checked the endpoint count and then compared m_discoveryServerEndpoints[ii] against serverEndpoints[ii] by index. It also compared UserIdentityTokens[jj] by index within each endpoint. As a result, a server that returned the same legal endpoints in a different order, or returned the same token policies in a different order within an endpoint, could be rejected with BadSecurityChecksFailed even though the endpoint data itself had not changed.

Changes

• Replace index-based ServerEndpoints validation with unordered matching based on the same endpoint fields already validated by the client.
• Compare UserIdentityTokens as an unordered multiset instead of requiring the server to preserve the original token enumeration order.
• Extend the client session test scaffolding so discovery endpoints can be supplied explicitly.
• Add regression tests that verify OpenAsync() accepts reordered ServerEndpoints and reordered UserIdentityTokens.

[codecov]

Codecov Report

❌ Patch coverage is 67.44186% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.54%. Comparing base (84fa8e8) to head (a0241ff).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Opc.Ua.Client/Session/Session.cs	67.44%	9 Missing and 5 partials ⚠️

❌ Your patch check has failed because the patch coverage (67.44%) 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             @@
##           master    #4029      +/-   ##
==========================================
- Coverage   73.85%   73.54%   -0.32%     
==========================================
  Files        1345     1345              
  Lines      179988   180056      +68     
  Branches    31668    31681      +13     
==========================================
- Hits       132938   132415     -523     
- Misses      36299    36928     +629     
+ Partials    10751    10713      -38     

Files with missing lines	Coverage Δ
src/Opc.Ua.Client/Session/Session.cs	72.81% <67.44%> (+0.88%)	⬆️

... and 47 files with indirect coverage changes

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[marcschier]

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