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.
• Use an isolated temporary SQLite database for each test factory.
• Clean up temporary SQLite database files after the test run.
• 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


## Build note

The current upstream branch reports NU1903 for the vulnerable transitive
SQLite dependency and treats all warnings as errors.

This PR keeps NU1903 visible but excludes it from `TreatWarningsAsErrors`
so the functional-test changes can be built and verified.

The actual dependency update is handled separately in #1067.