Add injectable IScenarioStateStore for distributed scenario state (#1430)

* Move ScenarioState to Abstractions and add IScenarioStateStore interface

ScenarioState is moved to the Abstractions project so it can be referenced
by the new IScenarioStateStore interface. The interface defines the contract
for storing and retrieving scenario states, enabling distributed implementations.

* Add InMemoryScenarioStateStore default implementation

Wraps ConcurrentDictionary with OrdinalIgnoreCase comparer, preserving
exact current behavior. The Update method encapsulates read-modify-write
so distributed implementations can make it atomic.

* Wire IScenarioStateStore into middleware options, settings, and consumers

Replace direct ConcurrentDictionary<string, ScenarioState> usage with
IScenarioStateStore across all consumer files. The store is injectable
via WireMockServerSettings.ScenarioStateStore, defaulting to the
InMemoryScenarioStateStore for backward compatibility.

* Add FileBasedScenarioStateStore for persistent scenario state

In-memory ConcurrentDictionary backed by JSON file persistence in
__admin/scenarios/. Reads from cache, mutations write through to disk.
Constructor loads existing state from disk on startup.

* Make ScenarioStateStore non-nullable with default InMemoryScenarioStateStore

Move InMemoryScenarioStateStore from WireMock.Net.Minimal to
WireMock.Net.Shared so it lives alongside WireMockServerSettings.
This allows WireMockServerSettings.ScenarioStateStore to be
non-nullable with a default value, following the same pattern as
DefaultJsonSerializer. The null-coalescing fallback in
WireMockMiddlewareOptionsHelper is no longer needed.

src/WireMock.Net.Minimal/Handlers/FileBasedScenarioStateStore.cs

@@ -0,0 +1,131 @@
+// Copyright © WireMock.Net
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Linq;
+using Newtonsoft.Json;
+
+namespace WireMock.Handlers;
+
+public class FileBasedScenarioStateStore : IScenarioStateStore
+{
+    private readonly ConcurrentDictionary<string, ScenarioState> _scenarios = new(StringComparer.OrdinalIgnoreCase);
+    private readonly string _scenariosFolder;
+    private readonly object _lock = new();
+
+    public FileBasedScenarioStateStore(string rootFolder)
+    {
+        _scenariosFolder = Path.Combine(rootFolder, "__admin", "scenarios");
+        Directory.CreateDirectory(_scenariosFolder);
+        LoadScenariosFromDisk();
+    }
+
+    public bool TryGet(string name, [NotNullWhen(true)] out ScenarioState? state)
+    {
+        return _scenarios.TryGetValue(name, out state);
+    }
+
+    public IReadOnlyList<ScenarioState> GetAll()
+    {
+        return _scenarios.Values.ToArray();
+    }
+
+    public bool ContainsKey(string name)
+    {
+        return _scenarios.ContainsKey(name);
+    }
+
+    public bool TryAdd(string name, ScenarioState scenarioState)
+    {
+        if (_scenarios.TryAdd(name, scenarioState))
+        {
+            WriteScenarioToFile(name, scenarioState);
+            return true;
+        }
+
+        return false;
+    }
+
+    public ScenarioState AddOrUpdate(string name, Func<string, ScenarioState> addFactory, Func<string, ScenarioState, ScenarioState> updateFactory)
+    {
+        lock (_lock)
+        {
+            var result = _scenarios.AddOrUpdate(name, addFactory, updateFactory);
+            WriteScenarioToFile(name, result);
+            return result;
+        }
+    }
+
+    public ScenarioState? Update(string name, Action<ScenarioState> updateAction)
+    {
+        lock (_lock)
+        {
+            if (_scenarios.TryGetValue(name, out var state))
+            {
+                updateAction(state);
+                WriteScenarioToFile(name, state);
+                return state;
+            }
+
+            return null;
+        }
+    }
+
+    public bool TryRemove(string name)
+    {
+        if (_scenarios.TryRemove(name, out _))
+        {
+            DeleteScenarioFile(name);
+            return true;
+        }
+
+        return false;
+    }
+
+    public void Clear()
+    {
+        _scenarios.Clear();
+
+        foreach (var file in Directory.GetFiles(_scenariosFolder, "*.json"))
+        {
+            File.Delete(file);
+        }
+    }
+
+    private string GetScenarioFilePath(string name)
+    {
+        var sanitized = string.Concat(name.Select(c => Path.GetInvalidFileNameChars().Contains(c) ? '_' : c));
+        return Path.Combine(_scenariosFolder, sanitized + ".json");
+    }
+
+    private void WriteScenarioToFile(string name, ScenarioState state)
+    {
+        var json = JsonConvert.SerializeObject(state, Formatting.Indented);
+        File.WriteAllText(GetScenarioFilePath(name), json);
+    }
+
+    private void DeleteScenarioFile(string name)
+    {
+        var path = GetScenarioFilePath(name);
+        if (File.Exists(path))
+        {
+            File.Delete(path);
+        }
+    }
+
+    private void LoadScenariosFromDisk()
+    {
+        foreach (var file in Directory.GetFiles(_scenariosFolder, "*.json"))
+        {
+            var json = File.ReadAllText(file);
+            var state = JsonConvert.DeserializeObject<ScenarioState>(json);
+            if (state != null)
+            {
+                _scenarios.TryAdd(state.Name, state);
+            }
+        }
+    }
+}