tests: stop force-loading plugin into AppDomain - lets PluginLoader register MVC parts

The actual root cause of master CI #22's OTP-404 failure (and the
v1.1.0-cycle GetService<TPluginService> null result I worked around
with HTTP-only testing): the fixture ctor was force-loading the
plugin assembly into the AppDomain via typeof(SamRockProtocolPlugin).

PluginManager.AddPlugins at submodules/btcpayserver/BTCPayServer/
Plugins/PluginManager.cs:155 scans AppDomain.CurrentDomain.GetAssemblies()
FIRST via PreloadPluginsFromAssemblies and registers any plugin it
finds there with Loader=null (line 135). Later when the DEBUG_PLUGINS
/ plugins-folder path tries to add the same plugin via
PluginLoader.CreateFromAssemblyFile, the dedup check at line 204
(if preloadedPlugins.Contains(identifier) continue) skips it.

The AppDomain-scanned plugin then takes the Loader=null branch at
line 246-256: AddAssemblyLoadContexts is skipped, GetPluginInstanceFromAssembly
still runs (so the IBTCPayServerPlugin shows in DI), but
mvcBuilder.AddPluginLoader(loader) at line 256 is SKIPPED because
Loader is null. MVC ApplicationParts never includes the plugin's
assembly, controllers never register, all routes 404.

Fix: let the file-based DEBUG_PLUGINS / plugins-folder path resolve
the plugin first via PluginLoader (Loader non-null), which triggers
mvcBuilder.AddPluginLoader and registers the controller assembly
with MVC. Remove the typeof() force-load that was sabotaging this.

Also reverts the LoadPluginsInDefaultAssemblyContext attempt - that
property exists on later BTCPay versions but not on the v2.1.6-20
submodule SamRockProtocol pins, so it doesn't compile here.

The retry-with-backoff in the previous commit on this branch stays
as cheap defense for any residual timing window, but the real fix is
removing the force-load.

Author: r1ckstardev

SamRockProtocol.Tests/SharedPluginTestFixture.cs

@@ -13,10 +13,15 @@ public ConfigurablePluginTestFixture(string testDirName = "SharedPluginTests", b
     {
         _testDirName = testDirName;
         _useNewDb = useNewDb;
-        // Force-load the SamRockProtocol assembly into the AppDomain so
-        // PluginManager.PreloadPluginsFromAssemblies discovers it via
-        // AppDomain.CurrentDomain.GetAssemblies() before BTCPay startup.
-        _ = typeof(SamRockProtocol.SamRockProtocolPlugin);
+        // Do NOT force-load the SamRockProtocol assembly into the AppDomain
+        // here. PluginManager.AddPlugins scans AppDomain assemblies first and
+        // registers any plugin it finds with Loader=null. Plugins loaded via
+        // that path skip mvcBuilder.AddPluginLoader at PluginManager.cs:255,
+        // which means MVC ApplicationParts never includes the plugin's
+        // assembly -> the IBTCPayServerPlugin shows in DI but its controller
+        // routes 404. Let the DEBUG_PLUGINS / plugins-folder path resolve the
+        // plugin via PluginLoader.CreateFromAssemblyFile so the Loader is
+        // non-null and MVC integration happens.
     }
 
     public ServerTester ServerTester { get; private set; }
@@ -37,15 +42,6 @@ public void Initialize(UnitTestBase testInstance)
 
             var testDir = Path.Combine(Directory.GetCurrentDirectory(), _testDirName);
             ServerTester = testInstance.CreateServerTester(testDir, _useNewDb);
-            // BTCPay defaults plugins to isolated AssemblyLoadContext so production
-            // plugins can be unloaded/swapped without restarting. In tests the
-            // isolated context makes the plugin's assembly invisible to the test
-            // process's MVC ApplicationParts discovery, so the controllers never
-            // route (the plugin shows in DI but POSTs to its endpoints 404).
-            // Load into the default context so MVC can discover the controllers.
-            // Mirrors rockstardev/btcPayServerPlugins.RockstarDev's test fixture
-            // and closes the master CI #22 OTP-404 failure deterministically.
-            ServerTester.PayTester.LoadPluginsInDefaultAssemblyContext = false;
             ServerTester.StartAsync().GetAwaiter().GetResult();
         }
     }