Stop registering IStudentResultsClient in the worker's dependency bundle - #328

Closes #326. Remedial work on AB#296648.

The rules-engine worker crash-loops at startup on current main — exit 139,
restarting continuously, no queue consumer and no retention job running.

Unhandled exception. System.AggregateException: Some services are not able to be
constructed (Error while validating the service descriptor 'ServiceType:
IStudentResultsClient Lifetime: Scoped ImplementationType: StudentResultsBlobClient':
Unable to resolve service for type 'Microsoft.Extensions.Caching.Memory.IMemoryCache'
while attempting to activate 'StudentResultsBlobClient'.)

Cause

8f89b358 added IStudentResultsClient to AddInfrastructureDependencies.
StudentResultsBlobClient takes an IMemoryCache, and the worker has none:
AddMemoryCache comes from AddPersistenceDependencies, which the worker
deliberately opts out of so its manual DbContext registration stays the single
source of truth. The container is validated on build, so the whole process dies
rather than one feature failing — the queue consumer, the dead-letter, metrics,
search-analytics and content-staging retention jobs all stop, over a service
none of them uses.

This is the same shape as #308: a registration added for the web's benefit
landing in a bundle the worker also executes.

Why removing it is safe

AddInfrastructureDependencies has one production caller — the worker — and it
never resolves IStudentResultsClient; there is no reference to the type
anywhere in that project. Its only other caller is AnalyticsRegistrationTests,
which resolves IAnalyticsService alone through an unvalidated provider.

Every consumer — JourneyController, ResultSuggestionsController,
DevDataSeedingOrchestrator and SeedStudentResults — is in the web host,
which does not call that bundle at all and registers the client itself in
AddCpdBlobStorage.

BlobStorageExtensionsTests was added under AB#296648 for the mirror image of
this failure: the client registered only in Infrastructure, so the web could not
boot. The web-side registration fixed that; the Infrastructure one was left
behind, and it is what now takes the worker down. That test still passes here,
including its IStudentResultsClient case, so the incorrect-grade enquiry
journey is untouched.

Guard

Removing one line fixes this instance. The test added alongside it covers the
class: it builds the bundle with ValidateOnBuild and fails if anything in it
cannot be constructed from what the worker provides, which is exactly what the
host does at startup and exactly what the worker died doing.

The Build workflow cannot catch this on its own — it compiles and runs tests and
never starts the worker host — so without a guard the failure reaches whoever
runs the container next, which is how it was found.

Writing the test also surfaced that the bundle's blob clients need a storage
connection string before BlobServiceClient is registered. Compose and every
deployed environment supply one, so the test supplies one too rather than
asserting against a container shape no host has.

Testing

Full Release build of the solution, clean with no new warnings. 4,123 unit and
653 integration tests pass on the branch merged with current main.

The guard was written before the fix and failed with the same IMemoryCache
error the container showed; restoring the registration turns it red again.

Verified in the container: the worker starts with 0 restarts and no unhandled
exception, /healthz/live and /healthz/ready both return 200, rules load and
the retention jobs run. The web app serves 200 throughout.