Add unit tests for async, context, bulk, interceptors, and mapping

[cursor]

Problem

The test suite has ~221 tests, but several newer subsystems have no dedicated test coverage:

Area	Files
Async API	AsyncSqlClient, AsyncLinqHelper
EF-like context	DbContext, DbSet, ChangeTracker
Bulk operations	BulkOperations
Interceptors	LoggingInterceptor, AuditInterceptor, SoftDeleteInterceptor
Entity mapping	EntityMapping, FluentConfiguration, EntityMappingResolver
Transactions	DbTransaction

Proposed solution

Add focused unit tests per subsystem. Prefer tests that do not require a live database where possible (mock ISqlClient / IAsyncSqlClient, test SQL generation, state transitions).

Suggested test files:

• DBToolsUnitTest/Core/AsyncSqlClientTests.cs
• DBToolsUnitTest/Context/DbContextTests.cs
• DBToolsUnitTest/Bulk/BulkOperationsTests.cs
• DBToolsUnitTest/Interceptors/InterceptorTests.cs
• DBToolsUnitTest/Mapping/EntityMappingTests.cs
• DBToolsUnitTest/Core/DbTransactionTests.cs

Acceptance criteria

• Each listed subsystem has at least basic happy-path and error-path tests
• Tests run in CI without a live database (or are clearly categorized as integration)
• No significant regressions in existing test pass rate

Context

ProviderDialectTests is a good model for provider-agnostic unit tests without DB connectivity.