Move Guardian thread context into guardianv2 configuration - #43104

## What changed

Replace `features.guardian_thread_context` with `features.guardianv2.thread_context` and update the configuration schema. The setting controls thread-owned context for synchronous and asynchronous Guardian, defaults to `false`, and remains independent of `features.guardianv2.enabled`.

## Testing

Update configuration tests to cover the nested setting, profile overrides, and enabling thread context while Guardian v2 is disabled. Migrate Guardian approval, history, and async scorer tests to the new configuration key.

GitOrigin-RevId: 54a91555eb68001e0adedc8a07bdbc5ea2273f9b