feat(dcl): move the OPC UA client to OPC Foundation SDK 2.0.1 (#125)

OPCFoundation.NetStandard.Opc.Ua.Client and the test-only .Server move
1.5.378.182 -> 2.0.1, the version OtOpcUa master runs (lmxopcua#782; #799
and #801 closed). Transitive Core/Core.Types/Types/Configuration/
Security.Certificates resolve to 2.0.1.

API breaks fixed in the DCL adapter:
- Built-in types are immutable structs (Variant, DataValue, NodeId,
  LocalizedText, StatusCode, ArrayOf<T>). New OpcUaValueBoundary restores
  the SDK 1.5 CLR shapes for every read, subscription and alarm-event
  value (byte[], UTC DateTime with no-timestamp = MinValue, Guid,
  detached arrays) and builds write variants with SDK 1.5's
  new Variant(object) semantics (byte[] = ByteString scalar).
- StatusCodes members are no longer constants: switches/patterns over
  them (verify-outcome mapping, connection-level classification,
  BadResourceUnavailable) match by value.
- CertificateIdentifier no longer caches a certificate and the
  CertificateValidator/CertificateValidation event is gone: the
  configuration's CertificateManager owns trust lists and the loaded
  application certificate and is disposed with the session; auto-accept
  and the verify probe's capture-and-reject use AcceptError.
- new UserIdentity(X509Certificate2) is gone: X509 user identities use
  X509IdentityTokenHandler with an on-demand PFX provider (still fails
  fast on a missing file/key).
- DefaultSessionFactory takes the telemetry context; collections are
  ArrayOf<T>; EventFilter select clauses and ContentFilter operands are
  built immutably.

UserName identity (lmxopcua#880): SDK 2's default handler sends a
password over 64 bytes - every 65-character OtOpcUa API key - as an
RsaEncryptedSecret that SDK 1.5 servers reject with BadIdentityTokenInvalid
and SDK 2 servers accept only within a 5-minute clock window (#873). The
DCL now always uses LegacyRsaUserNameIdentityTokenHandler (ported from
OtOpcUa be28aa44), i.e. exactly what it sent on SDK 1.5, so it logs in to
both server generations during a mixed rollout.

Tests: the in-process secured test server is ported to SDK 2 and can
admit an API key; new tests cover the handler (mutation-checked: 5 of 7
fail with the SDK default handler), the value boundary, and a 65-char key
end to end over None/Sign/SignAndEncrypt with a wrong key reaching the
key check (BadIdentityTokenRejected).

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/OpcUaSecurityDowngradeTests.cs

@@ -13,7 +13,7 @@ public class OpcUaSecurityDowngradeTests
         await Assert.ThrowsAsync<InvalidOperationException>(() => RealOpcUaClient.DiscoverEndpointAsync("opc.tcp://test:4840",
             MessageSecurityMode.SignAndEncrypt, _ => discoveryFails
                 ? throw new Exception("discovery unavailable")
-                : Task.FromResult(new EndpointDescriptionCollection { new("opc.tcp://test:4840") { SecurityMode = MessageSecurityMode.None } })));
+                : Task.FromResult<IReadOnlyList<EndpointDescription>>([new("opc.tcp://test:4840") { SecurityMode = MessageSecurityMode.None }])));
     }
     /// <summary>Verifies that DC4: exact requested mode is selected.</summary>
     [Fact]
@@ -21,6 +21,6 @@ public class OpcUaSecurityDowngradeTests
     {
         var encrypted = new EndpointDescription("opc.tcp://test:4840") { SecurityMode = MessageSecurityMode.SignAndEncrypt };
         Assert.Same(encrypted, await RealOpcUaClient.DiscoverEndpointAsync("opc.tcp://test:4840", MessageSecurityMode.SignAndEncrypt,
-            _ => Task.FromResult(new EndpointDescriptionCollection { new("opc.tcp://test:4840"), encrypted })));
+            _ => Task.FromResult<IReadOnlyList<EndpointDescription>>([new("opc.tcp://test:4840"), encrypted])));
     }
 }