Tests: route Yavsc.Org test host through Testing environment

TestWebApplicationFactory used ASPNETCORE_ENVIRONMENT=Development, which
caused Program.Main's AddConfiguration("org") to load the tracked
appsettings-org.json (the reference file with the
'*** via dotnet user-secrets ou variable d'environnement ***'
placeholder connection string). Npgsql then failed to parse that
placeholder during host startup, failing six integration tests
(observed 2026-07-11: System.ArgumentException on
NpgsqlConnectionStringBuilder.set_Item).

Switching the test host to a dedicated Testing environment makes
AddConfiguration("org") pick up the new optional
appsettings-org.Testing.json file as the last source in the chain
(JSON → env vars), which overrides YavscConnection with the
InMemory marker and the Smtp section with the test stub values.

The .gitignore exception whitelists this file explicitly: it is a
configuration source for the test host, not a secrets file.

The WebServerFixture path is unchanged — it owns its
WebApplicationBuilder and adds the same in-memory override via its
BuildApp hook.

src/Yavsc.Org.Tests/TestWebApplicationFactory.cs

@@ -26,10 +26,15 @@ public class TestWebApplicationFactory : WebApplicationFactory<Program>
 {
     protected override void ConfigureWebHost(IWebHostBuilder builder)
     {
-        // UseDevelopmentEnvironment triggers the dev signing credential
-        // path in the production startup, so we don't need a real cert
-        // to satisfy IdentityServer at boot.
-        builder.UseEnvironment("Development");
+        // UseEnvironment("Testing") puts the host in a dedicated
+        // configuration environment so AddConfiguration("org") in
+        // Program.Main loads the optional appsettings-org.Testing.json
+        // file (which overrides the connection string and SMTP section
+        // for the test host). See that file for the values.
+        // We don't use "Development" because that environment is also
+        // used by the dev launcher and would change the signing
+        // credential path in IdentityServer; "Testing" is unambiguous.
+        builder.UseEnvironment("Testing");
 
         builder.ConfigureTestServices(services =>
         {

src/Yavsc.Org/appsettings-org.Testing.json

@@ -0,0 +1,11 @@
+{
+  "ConnectionStrings": {
+    "YavscConnection": "InMemory"
+  },
+  "Smtp": {
+    "Host": "smtp.test.local",
+    "Port": 465,
+    "UserName": "test-user",
+    "Password": "test-pass"
+  }
+}