fix: prevent conversation sync failures from breaking response delivery

[iamemilio]

Summary

When a response is created with a conversation parameter, _create_streaming_response syncs the input and output items to the conversation after inference completes. Previously, if this sync failed (e.g. the conversation store was temporarily unavailable or add_items raised), the exception propagated through the streaming generator. For non-streaming callers this meant losing the completed response entirely; for streaming callers the response.completed event was never yielded. In both cases the client lost a successfully generated response because of a secondary side-effect failure.

This PR wraps the conversation sync block in a try/except so that sync failures are logged with the full traceback but don't prevent the client from receiving its response. The _sync_response_to_conversation method itself still raises, so callers that invoke it directly retain the original behavior.

Test plan

• New test_sync_failure_still_yields_completed_response passes with the fix and fails without it
• Existing test_sync_response_to_conversation_api_error still passes (method itself still raises)
• All 9 conversation tests pass

[iamemilio]

Closing: swallowing the sync error would cause silent conversation context loss. The original behavior (propagating the exception) is correct — the client needs to know when a conversation write fails.