Adopt Weasel IDatabaseCleaner<T> and BatchedQuery EF Core APIs (#2473, #2478)

[jeremydmiller]

Summary

Closes #2473 and #2478.

#2473 – IDatabaseCleaner<TContext> in EF Core integration tests

Adopts Weasel 8.13.0's IDatabaseCleaner<TContext> and IInitialData<TContext> APIs to replace ad-hoc manual cleanup code in integration tests.

• database_cleaner_tests.cs – new integration-test file demonstrating:
  • services.AddDatabaseCleaner<ItemsDbContext>() (singleton, discovers tables from DbContext.Model, generates FK-safe deletion SQL: TRUNCATE CASCADE on PostgreSQL, DELETE FROM on SQL Server)
  • services.AddInitialData<ItemsDbContext, SeedItemsForTests>() for seed data that re-runs after ResetAllDataAsync()
  • Tests for DeleteAllDataAsync() (clears all rows) and ResetAllDataAsync() (clears then re-seeds)

#2478 – Weasel BatchedQuery codegen infrastructure

Adds the code-generation frames and integration tests for Weasel's BatchedQuery API, which batches multiple EF Core SELECT queries into a single database round trip.

• BatchedLoadEntityFrame – SyncFrame that enqueues a batch.QuerySingle(context.Set<T>().Where(…)) into a shared BatchedQuery instead of calling DbContext.FindAsync. Participates in a shared batch when a CreateBatchQueryFrame is upstream.
• CreateBatchQueryFrame – inserts a var batchQuery = context.CreateBatchQuery() variable that downstream load frames share.
• ExecuteBatchQueryFrame – issues await batchQuery.ExecuteAsync(ct) and then awaits each pending entity Task.
• batch_query_tests.cs – integration tests against SQL Server showing:
  • Two QuerySingle calls in one round trip
  • Query<T> list loading in a batch
  • Mixed single + list queries in the same batch
  • Unit tests for all three new codegen frame types

Test plan

• EfCoreTests project builds cleanly (dotnet build --framework net9.0)
• database_cleaner_usage_tests integration tests pass against SQL Server (requires docker compose up -d)
• batch_query_tests integration tests pass against SQL Server
• batched_load_entity_frame_codegen_tests unit tests pass without infrastructure

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