refactor(ai): make OpenAI Responses extend the Open Responses baseline #38537

Problem

The current generic Responses adapter layers on top of the OpenAI-specific Responses protocol:

• OpenAICompatibleResponses.route.protocol reuses OpenAIResponses.protocol
• the baseline contract therefore inherits OpenAI-specific request fields, events, options, and behavior

That dependency points in the wrong direction for a provider-neutral Open Responses protocol. The baseline should define the shared contract, while OpenAI Responses should extend it with OpenAI-specific capabilities.

Desired direction

• Define a provider-neutral Open Responses protocol as the baseline.
• Have OpenAI Responses extend or compose that baseline with OpenAI-only fields and behavior.
• Keep shared lowering and stream parsing in the baseline where the protocol guarantees it.
• Keep OpenAI-specific options, hosted tools, event variants, and transport behavior in the OpenAI layer.
• Revisit public names while making this split so the generic protocol and OpenAI-compatible provider adapter are not conflated.

Done when

• The protocol dependency points from OpenAI Responses to the generic Open Responses baseline, not the reverse.
• Generic providers can use the baseline without inheriting OpenAI-only behavior.
• Existing OpenAI Responses HTTP/WebSocket behavior remains covered by tests.
• The package exports and naming clearly distinguish the protocol contract from provider compatibility/configuration helpers.

Source discussion: team Slack thread