test(bff): /health returns 200 Healthy (refs #28)

Add the BFF Api/Tests skeleton (.NET 10, ASP.NET Core minimal API, xUnit
+ WebApplicationFactory) and a failing test asserting GET /health returns
200 with a Healthy body. The endpoint does not exist yet — the test fails
with NotFound, establishing red before the implementation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "10.0.203",
+    "rollForward": "latestFeature"
+  }
+}