cosmos-provider

# Cosmos DB Provider Non-relational provider with its own parallel query pipeline. Uses JSON for document materialization. ## Key Differences from Relational - No migrations — use `EnsureCreated()` - Documents as JSON — owned and complex types become embedded objects - Partition key configuration required for performance - `ETag` for optimistic concurrency - No cross-container joins ## Azure Cosmos DB Emulator for Tests - `TestEnvironment.InitializeAsync()` auto-starts a `Testcontainers.CosmosDb` container when `Test__Cosmos__DefaultConnection` is not set. Set the env var to use an existing emulator instead. - Skip tests requiring unsupported features on the Linux emulator with ` [ConditionalFact(typeof(CosmosTestEnvironment), nameof(CosmosTestEnvironment.IsNotLinuxEmulator))]`.