Adds the test project that the next commit will use to assert the
blog API endpoints. The fixture inherits from the shared
WebHostFixture (commit "refactor: extract WebHostFixture…") and
wires up only the bits the blog API needs:
* In-memory ApplicationDbContext — BlogSpotService is used as-is,
no mock. The first tests will exercise the real service against
an empty table.
* Trivial IFileSystemAuthManager stub (the GET index path never
reads the file system).
* TestAuthPolicyProvider swapped in, so X-Test-Role satisfies
[Authorize("BlogScope")].
Two smoke tests verify the fixture boots and the controller
pipeline is reachable. The first behavioural test
(GET /api/v1/blog → 200) lands in the next commit.
Also promotes two xunit.v3.* package versions to the root
Directory.Packages.props so future test projects can share them.
