Speed up DatabaseTests by reusing the SQL container #1889

The database test suite is currently dominated by repeated SQL container startup in TransactionProcessor.DatabaseTests/BaseTest.cs. Each test instance creates its own SQL container and runs EnsureCreatedAsync, which makes the suite much slower than necessary even though each test already uses a unique database name.

Goal:

• Reuse a single SQL Server container across the DatabaseTests suite
• Keep one isolated database per test
• Preserve all existing assertions and coverage

Planned work:

1. Introduce a shared xUnit fixture for SQL Server lifecycle.
2. Move BaseTest onto that fixture so tests keep unique TestId-based database names but do not start a container per test.
3. Update the existing test classes to use the shared fixture.
4. Validate the runtime improvement on the two heaviest classes: MerchantEventTests and TransactionEventTests.

Important constraint:

• Do not share a database between tests. Isolation must stay intact.