FIX: Preserve DatasetConfiguration subclass when backend overrides dataset_names

[varunj-msft]

Description

When dataset_names is passed through the backend (ScenarioRunService._build_init_kwargs), we used to always construct a plain DatasetConfiguration. That silently dropped subclass-specific behavior — most notably EncodingDatasetConfiguration.get_all_seed_attack_groups(), which shapes each seed into a SeedAttackGroup with a synthetic objective.

For garak.encoding this surfaced as a confusing runtime error during attack construction:

ValueError: SeedAttackGroup must have exactly one objective. Found 0.

Reproducible end-to-end against the real garak_slur_terms_en dataset.

Fix: when dataset_names is supplied, build a fresh instance of the scenario's own default-dataset-config class so subclass overrides are preserved. If a future subclass adds required init kwargs we can't populate, fall back to the plain DatasetConfiguration with a logged warning so the operator has a trail.

The max_dataset_size-only path is unchanged — it still mutates the throwaway introspection instance's default config.

First in a series of small PRs for the Standardizing Scenarios work . Lands ahead of the Encoding scenario standardization PR, which depends on this fix to make the documented fast path usable via the API.

Tests and Documentation

• 5 new regression tests in tests/unit/backend/test_scenario_run_service.py covering: subclass preservation with dataset_names, with dataset_names + max_dataset_size, with dataset_names only (no max), fallback to plain DatasetConfiguration when subclass init is incompatible (+ caplog assertion on the warning), and the introspection-failure path.
• All 5 new tests fail against pre-fix code; verified by reverting the prod change and rerunning.
• All 30 pre-existing tests in the file still pass.
• Full backend suite: 619 passed, 4 skipped.
• Full scenario suite: 624 passed.
• ruff check + ruff format --check + ty all clean on both touched files.
• No JupyText / notebook changes (backend service fix, no doc impact).