Share a single MongoClient and fix LiteDB TableCollection - #757

Type: bugfix

Issue

No issues were open for these; both came out of an audit of the data layer. Two independent changes,
one commit each. They are not of equal weight, so to be straight about it:

1. LiteDBRepository.TableCollection<C>() queried a collection named "T" — a real bug.
nameof(T) on a generic type parameter evaluates to the literal string "T", not the entity name:

return Database.GetCollection<C>(nameof(T)).Query().ToEnumerable().AsQueryable();

MongoRepository.TableCollection<C>() already used typeof(T).Name, so the two providers disagreed.

Reach is narrow and worth stating plainly: TableCollection<C>() has exactly one production
consumer, GetGenericQueryHandler in Grand.Module.Api. Triggering the bug needs the LiteDB
provider and the API module enabled, and both are opt-in. In that combination every generic API
query returned an empty result set.

2. A MongoClient was constructed per DI scope — hygiene, not a defect.
src/Core/Grand.Infrastructure/Startup/StartupApplication.cs registered the database as
AddScoped(_ => new MongoClient(clientSettings).GetDatabase(databaseName)), so a client was built
per scope, that is per request.

An earlier draft of this description called that the most impactful defect in the data layer. That
was wrong, and the corrected version is less dramatic: the driver deduplicates the underlying
cluster through ClusterRegistry, so this was not a connection pool per request, and
MongoClientSettings.FromConnectionString already ran once at startup. What actually happened on
every request was a settings clone and freeze, a ClusterKey computation, a lock-protected registry
lookup, and a client wrapper allocation.

That is small. It is still contrary to the driver's documented usage — MongoClient is meant to be a
single, thread-safe instance owning the connection pool and cluster monitoring — and removing it
costs four lines and changes no behaviour.

Solution

• Use typeof(T).Name in LiteDBRepository.TableCollection<C>(), matching the Mongo
  implementation, with a regression test in Grand.Data.Tests.
• Register IMongoClient as a singleton and resolve the scoped IMongoDatabase from it. Consumers
  are untouched — they still inject IMongoDatabase, still scoped.

On lifetime, since it is the one thing that could bite: previously the scoped clients were created
by a factory, so the container disposed one per scope. If MongoClient.Dispose tore down the shared
registry cluster, the application would fail continuously — it does not. The singleton is disposed
at shutdown, which is correct. Nothing else in the solution resolves IMongoClient.

Note for reviewers: the parameterless constructors of MongoRepository and MongoStoreFilesContext
still build their own MongoClient. DI only selects those before installation, when
IMongoDatabase is not registered yet — afterwards it picks the IMongoDatabase overloads. They
are deliberately left alone rather than widened into this change.

Breaking changes

None. IMongoClient was not registered before, so the new registration cannot collide, and no
public signature changed. The scoped IMongoDatabase registration is preserved, so every existing
consumer resolves exactly as it did.

Testing

1. dotnet build ./GrandNode.sln — succeeds.
2. dotnet test ./src/Tests/Grand.Data.Tests/Grand.Data.Tests.csproj — 46/46 pass.
3. dotnet test ./src/Tests/Grand.Infrastructure.Tests/Grand.Infrastructure.Tests.csproj — 84/84 pass.
4. To confirm the new test guards the defect, temporarily put nameof(T) back in
   LiteDBRepository.TableCollection<C>() and re-run step 2.
   TableCollection_ReadsTheEntityCollection fails with expected: 2, actual: 0. Revert.
5. Mongo runtime check: start the storefront against MongoDB and browse the catalogue, place an
   order, and open the admin panel. Everything resolving IMongoDatabase must behave as before —
   the point of the change is that only the client's lifetime moved.
6. LiteDB check: set Database:UseLiteDb to true, enable the Grand.Module.Api feature, and call
   a generic API query endpoint. It returns rows; before this change it returned an empty set.

Steps 5 and 6 have not been run here — they need a running instance and, for 6, a LiteDB
installation with the API module switched on.

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