feat: Add json_object response format (opt-in)

[ajianaz]

Summary

Add OpenAI-compatible response_format: json_object as opt-in via .cora.yaml config to reduce JSON parsing errors.

Accept Criteria

• Add review.response_format field to config schema (values: json_object, none)
• Pass response_format to LLM API when enabled
• Graceful fallback if provider returns error with response_format
• Default behavior unchanged (backward compatible)
• Unit test: ChatRequest serialization with response_format

Scope

Config schema + LLM request building + fallback handling

[ajianaz]

Closed in v0.1.6 release