Phase 3 PR 66 -- OPC UA Client (gateway) driver project scaffold + IDriver session lifecycle. First driver that CONSUMES OPC UA rather than PUBLISHES it -- connects to a remote server and re-exposes its address space through the local OtOpcUa server per driver-specs.md \u00A78. Uses the same OPCFoundation.NetStandard.Opc.Ua.Client package the existing Client.Shared ships (bumped to 1.5.378.106 to match). Builds its own ApplicationConfiguration (cert stores under %LocalAppData%/OtOpcUa/pki so multiple driver instances in one OtOpcUa server process share a trust anchor) rather than reusing Client.Shared -- Client.Shared is oriented at the interactive CLI with different session-lifetime needs (this driver is always-on, needs keep-alive + session transfer on reconnect + multi-year uptime). Navigated the post-refactor 1.5.378 SDK surface: every Session.Create* static is now [Obsolete] in favour of DefaultSessionFactory; CoreClientUtils.SelectEndpoint got the sync overloads deprecated in favour of SelectEndpointAsync with a required ITelemetryContext parameter. Driver passes telemetry: null! to both SelectEndpointAsync + new DefaultSessionFactory(telemetry: null!) -- the SDK's internal default sink handles null gracefully and plumbing a telemetry context through the driver options surface is out of scope (the driver emits its own logs via the DriverHealth surface anyway). ApplicationInstance default ctor is also obsolete; wrapped in #pragma warning disable CS0618 rather than migrate to the ITelemetryContext overload for the same reason. OpcUaClientDriverOptions models driver-specs.md \u00A78 settings: EndpointUrl (default opc.tcp://localhost:4840 IANA-assigned port), SecurityPolicy/SecurityMode/AuthType enums, Username/Password, SessionTimeout=120s + KeepAliveInterval=5s + ReconnectPeriod=5s (defaults from spec), AutoAcceptCertificates=false (production default; dev turns on for self-signed servers), ApplicationUri + SessionName knobs for certificate SAN matching and remote-server session-list identification. OpcUaClientDriver : IDriver: InitializeAsync builds the ApplicationConfiguration, resolves + creates cert if missing via app.CheckApplicationInstanceCertificatesAsync, selects endpoint via CoreClientUtils.SelectEndpointAsync, builds UserIdentity (Anonymous or Username with UTF-8-encoded password bytes -- the legacy string-password ctor went away; Certificate auth deferred), creates session via DefaultSessionFactory.CreateAsync. Health transitions Unknown -> Initializing -> Healthy on success or -> Faulted on failure with best-effort Session.CloseAsync cleanup. ShutdownAsync (async now, not Task.CompletedTask) closes the session + disposes. Internal Session + Gate expose to the test project via InternalsVisibleTo so PRs 67-69 can stack read/write/discovery/subscribe on the same serialization. Scaffold tests (OpcUaClientDriverScaffoldTests, 5 facts): Default_options_target_standard_opcua_port_and_anonymous_auth (4840 + None mode + Anonymous + AutoAccept=false production default), Default_timeouts_match_driver_specs_section_8 (120s/5s/5s), Driver_reports_type_and_id_before_connect (DriverType=OpcUaClient, DriverInstanceId round-trip, pre-init Unknown health), Initialize_against_unreachable_endpoint_transitions_to_Faulted_and_throws, Reinitialize_against_unreachable_endpoint_re_throws. Uses opc.tcp://127.0.0.1:1 as the 'guaranteed-unreachable' target -- RFC 5737 reserved IPs get black-holed and time out only after the SDK's internal retry/backoff fully elapses (~60s), while port 1 on loopback refuses immediately with TCP RST which keeps the test suite snappy (5 tests / 8s). 5/5 pass. dotnet build clean. Scope boundary: ITagDiscovery / IReadable / IWritable / ISubscribable / IHostConnectivityProbe deliberately NOT in this PR -- they need browse + namespace remapping + reference-counted MonitoredItem forwarding + keep-alive probing and land in PRs 67-69.

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests.csproj

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