Consolidate test fixtures into shared conftest.py

[riccardomerenda]

Problem

Test files independently recreate sample configs, corpus records, model responses, and evaluation results. This leads to duplication and inconsistency — a schema change requires updates in multiple test files.

Proposal

Create/extend tests/conftest.py with shared fixtures:

• sample_config — a minimal valid DriftcutConfig
• sample_corpus — a list of 5-10 PromptRecord covering all criticality/output type combinations
• sample_model_response — a ModelResponse factory
• sample_evaluation — a PromptEvaluation factory

Keep test-specific data local, move repeated structures to fixtures.

Impact

• Less duplication across 14 test files
• Easier to update when models/config schema changes
• Lower barrier for writing new tests