Add Testcontainers-based integration tests for multi-provider CI

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Problem

~80 tests are marked inconclusive/skipped in CI because they require a live SQL Server at 127.0.0.1:1433. There are no integration tests against PostgreSQL, MySQL, or SQLite.

Current CI (PR #24) passes unit tests but cannot validate end-to-end CRUD/LINQ behavior against real databases.

Proposed solution

1. Add Testcontainers for one or more providers in CI
2. Categorize tests with [TestCategory("Integration")] vs unit tests
3. Run integration tests in a separate CI job (or nightly) with service containers
4. Keep fast unit-only job for PR feedback

Suggested starting point:

• SQL Server container for existing DB-dependent tests
• SQLite in-memory or file-based container for lightweight multi-provider coverage

Acceptance criteria

• CI can run at least one integration test suite against a real database
• Unit test job remains fast (< 2 min)
• Integration tests are opt-in via category/filter
• README documents how to run integration tests locally

Related

Complements #28 (unit test coverage). Reduces reliance on SkipIfDatabaseUnavailable() / Assert.Inconclusive.