Add OpenAI Responses and Conversations support

[kaxil]

Add OpenAI Responses and Conversations support to the provider -- the interface OpenAI now recommends for text generation and tool use, and the forward path as the Assistants API approaches its 2026-08-26 removal.

Stacked on #69068 (requires openai>=2.37.0). Review the latest commit; the first commit is #69068 and drops out once it merges.

What's added

New OpenAIHook methods:

• Responses: create_response, get_response, delete_response, cancel_response
• Conversations: create_conversation, get_conversation, update_conversation, delete_conversation

New operator:

• OpenAIResponseOperator -- generates a response from an input_text prompt and returns the aggregated output text. Pass instructions, tools, conversation, previous_response_id etc. through response_kwargs.

Design notes

• Operator scope: the operator is intentionally synchronous and returns output_text (the common case). For previous_response_id chaining, background=True responses, or the full structured Response, use the hook directly -- documented in the operator docstring and the hook guide. It logs a warning when the response status isn't completed so an empty output_text (for example from a queued background response) isn't a silent success.
• Conversation items (conversations.items) are out of scope here; multi-turn continuity works via conversation=<id> / previous_response_id on responses.

Follow-up

• Deprecate OpenAI Assistants and Threads hook methods #69071

Related:

• Update OpenAI provider to OpenAI Python SDK 2.x #69068
• Add Workload Identity authentication to OpenAI provider #69069
• Deprecate OpenAI Assistants and Threads hook methods #69071

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