fix: normalize compacted Responses user inputs before session reuse

[seratch]

This pull request fixes replay safety for compacted Responses user messages in OpenAIResponsesCompactionSession.

When responses.compact() returns user input_image or input_file content with null alternatives such as file_id: null, the SDK was previously storing that shape back into session history unchanged. That left the next responses.create call dependent on server tolerance for contract-invalid null combinations and also meant a normalization failure could clear the underlying session before replacement succeeded.

This change normalizes compacted user image and file inputs into replay-safe shapes before they are written back to the session, keeping only valid source fields such as image_url, file_id, file_url, or file_data as appropriate. It also defers clearing the underlying session until normalization succeeds so malformed compacted output does not erase existing history. The update includes regression coverage for compacted image normalization, compacted file normalization, and the failure path that preserves prior session items.

see also: openai/openai-agents-js#1181