Make SQLite fallback reliable when Docker is unavailable

[MartinHock]

Summary

• Make the functional test database provider explicitly selectable.
• Keep SQL Server through Testcontainers as the preferred provider.
• Fall back to SQLite only when Docker is unavailable.
• Avoid failing the complete test run when Docker-specific coverage cannot run.
• Use an isolated temporary SQLite database for each test factory.
• Clean up the temporary SQLite database files after the test run.
• Expose the active database provider for diagnostics and assertions.
• Add tests that verify the SQLite provider and a functional endpoint using SQLite.

Behavior

When Docker is available:

• SQL Server Testcontainers are used.
• All functional tests run with SQL Server.
• The Docker-specific test runs normally.

When Docker is unavailable:

• The functional test factory falls back to SQLite.
• The Docker-specific test is skipped.
• The remaining functional tests continue to pass.

Only DockerUnavailableException triggers the fallback or skipped test. Other container or SQL Server startup errors remain visible as test failures.

Verification

With Docker running:

Total tests: 6
Passed: 6
Failed: 0
Skipped: 0

With Docker stopped:

Total tests: 6
Passed: 5
Failed: 0
Skipped: 1

Closes #1028