Add sub state machines and the materialized Part 16 model to the StateMachines sample (#829) (#860)

The sample was written against 2.0.0-preview.2, whose fluent builder
materialized no state or transition nodes and could not attach a sub state
machine to a state. preview.4 does both, so the two parts issue #829 deferred
can be built.

Server: the Operation machine now carries a Production sub state machine below
its Running state - Loading (initial), Processing and Unloading, with StartBatch
as a cause of the child and the rest of the batch on the child's own timers. The
framework activates and resets it when the parent enters Running and suspends it
when the parent leaves. The states, transitions, their numbers and the Part 16
references are materialized by the builder, so the server code only declares the
machine.

Client: the window lists the model of the machine - a row per state and per
transition with its number and NodeId - read through GetAvailableStatesAsync and
GetAvailableTransitionsAsync, finds the sub state machine with
GetSubStateMachineAsync on the state node it hangs off, and watches the Operation
machine through ObserveEffectiveStateAsync so a transition of the parent or of
the child is reported as one combined snapshot. The cause of the child is offered
only while the child is active, which is what its Executable attribute says.

Tests: the browse shape assertion covers the new children; new cases hold the
machine to AvailableStates / AvailableTransitions in declaration order with the
declared numbers, to the FromState / ToState / HasCause / HasEffect references of
a transition, to HasSubStateMachine hanging off the state node rather than the
machine root, to Bad_StateNotActive and BadNotExecutable while the child is
suspended, and to a batch running to completion and starting over on re-entry.
The tier 2 client case checks the model list, the cause becoming executable and
the displayed sub state.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: romanett

README.md

@@ -85,8 +85,8 @@ These paired client/server samples each demonstrate a specific OPC UA feature se
 | [RoleManagement Client](Workshop/RoleManagement/Client) | **Client** | Connects as different accounts to see how the address space and the permissions change, and manages the RoleSet of the server. |
 | [SimpleEvents Server](Workshop/SimpleEvents/Server) | **Server** | Generates simple OPC UA events to demonstrate event subscriptions. |
 | [SimpleEvents Client](Workshop/SimpleEvents/Client) | **Client** | Subscribes to and displays simple events from an events server. |
-| [StateMachines Server](Workshop/StateMachines/Server) | **Server** | Demonstrates OPC UA Part 16 state machines: one declared with the fluent `StateMachineBuilder` (states, transitions, causes, guards, timed transitions) and the standard `ProgramStateMachineType` with behaviour attached to it. |
-| [StateMachines Client](Workshop/StateMachines/Client) | **Client** | Drives both state machines of a StateMachines server and streams their transitions as they happen. |
+| [StateMachines Server](Workshop/StateMachines/Server) | **Server** | Demonstrates OPC UA Part 16 state machines: one declared with the fluent `StateMachineBuilder` (states, transitions, causes, guards, timed transitions and a sub state machine below one of its states) and the standard `ProgramStateMachineType` with behaviour attached to it. |
+| [StateMachines Client](Workshop/StateMachines/Client) | **Client** | Drives both state machines of a StateMachines server, reads the states and transitions each of them declares, and streams the transitions of a machine and of its sub state machine as they happen. |
 | [UserAuthentication Server](Workshop/UserAuthentication/Server) | **Server** | Demonstrates OPC UA user authentication with username/password and certificate-based identity tokens. |
 | [UserAuthentication Client](Workshop/UserAuthentication/Client) | **Client** | Connects using different user identity tokens (anonymous, username, certificate) to a UserAuthentication server. |
 | [Views Server](Workshop/Views/Server) | **Server** | Demonstrates OPC UA Views — subsets of the address space exposed as named views. |

Tests/SampleClients.Tests/WorkshopClientSubscriptionTests.cs

@@ -365,6 +365,56 @@ private static async Task PowerOnTheMachineAsync(Form form, CancellationToken ct
                 Is.True,
                 "Start is declared for Idle, so the client has to offer it once the machine " +
                 "has been powered on.");
+
+            // the client also reads the model of the machine - a row per state and per
+            // transition - which a server materializes only if it exposes AvailableStates and
+            // AvailableTransitions.
+            Assert.That(
+                HasRows(form, "ModelLV"),
+                Is.True,
+                "The StateMachines client did not read the states and transitions of the " +
+                "machine, so the server does not publish them.");
+
+            // Running has a machine of its own below it, which is suspended until the parent
+            // gets there: the client offers its cause only once it is active.
+            var startBatch = (Button)WinFormsHarness.FindControl(form, "StartBatchBTN");
+
+            Assert.That(
+                startBatch,
+                Is.Not.Null,
+                "The StateMachines client no longer has a 'StartBatchBTN'. Rename it here too.");
+
+            Assert.That(
+                startBatch.Enabled,
+                Is.False,
+                "The sub state machine is not active while the machine is Idle, so none of " +
+                "its causes may be offered.");
+
+            SampleFormDriver.TryInvokeHandler(form, "OperationCauseBTN_ClickAsync", start);
+
+            bool producing = await WaitAsync(() => startBatch.Enabled, ct).ConfigureAwait(true);
+
+            Assert.That(
+                producing,
+                Is.True,
+                "Entering Running has to activate the sub state machine, and the client has " +
+                "to offer the cause of the machine it activated.");
+
+            Control production = WinFormsHarness.FindControl(form, "ProductionStateTB");
+
+            Assert.That(
+                production,
+                Is.Not.Null,
+                "The StateMachines client no longer has a 'ProductionStateTB'. Rename it here too.");
+
+            bool loading = await WaitAsync(() => production.Text == "Loading", ct)
+                .ConfigureAwait(true);
+
+            Assert.That(
+                loading,
+                Is.True,
+                "The client has to show the state of the sub state machine, which starts over " +
+                $"in Loading whenever the parent enters Running. It showed '{production.Text}'.");
         }
 
         /// <summary>