Enforce ownership across the OpenAI Conversations lifecycle

[leseb]

Parent epic: #732

Summary

Apply the trusted ownership context and constrained store boundary to the complete OpenAI Conversations lifecycle.

Covered Paths

• Conversation creation, retrieval, update, and deletion.
• Conversation-item creation, listing, retrieval, and deletion.
• Conversation use from a Responses request.
• Response append-back into a Conversation.
• Denormalized message-cache synchronization.

Requirements

• Bind each Conversation to the immutable owner active at creation.
• Authorize item operations through the parent Conversation boundary.
• Require the same owner for Conversation rehydration and append-back.
• Preserve the original owner during metadata updates and cache synchronization.
• Remove multi-tenant fallback to DEFAULT_TENANT_ID from covered paths.
• Return equivalent not-found behavior for missing and unauthorized resources.
• Ensure pagination cannot expose items from an unauthorized parent.
• Keep client-controlled Conversation metadata separate from ownership and future policy attributes.

Acceptance Criteria

• Same-owner lifecycle and item operations succeed.
• Same-tenant cross-owner lifecycle, item, rehydration, and append-back operations fail.
• Cross-tenant operations fail even when issuer and subject match.
• Direct item retrieval cannot bypass parent ownership.
• Pagination runs only after the parent Conversation has been authorized.
• Metadata updates, append-back, and cache synchronization cannot change ownership.
• OpenAI SDK integration tests cover owner isolation for Conversations and items.

Non-Goals

• Independent ownership for Conversation items.
• PPE resource-policy decisions.
• Database migration.