refactor: unify model caching on ResponseCache

[Luodian]

Summary

• remove provider-local JSON cache code paths from API/chat wrappers and rely on evaluator-level ResponseCache
• route generate_until_agentic through ResponseCache and normalize generated output handling
• harden cache keying by including request content hash for generation requests to prevent prompt collision on same doc/index
• add regression coverage for agentic cache reuse and same-doc different-prompt key separation

Validation

• uv run python -m unittest discover -s test/cache -p "test_*cache*.py"
• uv run python -m unittest discover -s test/models -p "test_model_registry_v2.py"

Refs: LMM-276