fix: preserve response hydration checkpoints

[franciscojavierarceo]

Summary

• Materialize ADR-02 response checkpoints by carrying prior history item IDs forward.
• Hydrate previous responses/conversations even when the new turn uses store=false, while keeping store=false turns unpersisted.
• Reject requests that provide both conversation_id and previous_response_id, and assert hydrated upstream request bodies in stateful tests.

Test Plan

• cargo test -p agentic-core
• cargo clippy -p agentic-core --all-targets -- -D warnings

[maralbahari]

@franciscojavierarceo thank you for catching the these. LGTM