Refactor: Inject MongoClient via DI as Singleton

[emuuu]

Context

Currently EntityContext<TEntity> and MongoRepositoryHealthCheck create MongoClient instances internally. In v10.2.0, a static ConcurrentDictionary<string, MongoClient> cache was added to avoid creating new clients per request, which solves the immediate connection pool waste problem.

However, the proper solution for v11 is to let the DI container manage MongoClient lifetime as a Singleton, allowing consumers to configure client settings (timeouts, SSL, etc.) in one place.

Proposed changes

• Register MongoClient as a Singleton in DI (keyed by connection string or named options)
• Inject MongoClient into EntityContext<TEntity> instead of creating it internally
• Inject MongoClient into MongoRepositoryHealthCheck
• Remove the static ConcurrentDictionary caches
• This is a breaking change (constructor signatures change)

Target version

v11.0.0