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/appsettings-org.Testing.json

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