test: make embedding tests offline

[ramonlimaramos]

Summary

• replace the shared unit-test embedding fixture with a deterministic offline provider
• keep SentenceTransformerProvider unit tests focused on the wrapper contract by stubbing model loading
• make provider auto-selection deterministic in embedding tests by clearing OPENAI_API_KEY

Validation

• uv run pytest tests/unit/test_embeddings.py -q
• uv run --python 3.11 --extra dev pytest tests/unit/test_embeddings.py -q
• uv run --python 3.11 --extra dev pytest tests/unit -q
• uv run --python 3.11 --extra dev pytest tests/ -v --cov=synapto --cov-report=term-missing
• uv run --python 3.11 --extra dev ruff check src/ tests/
• uv run --python 3.11 --extra dev bandit -r src/synapto/ -c pyproject.toml
• uv run --python 3.11 --extra dev pip-audit --ignore-vuln CVE-2025-3000