Add blank input when when no openai_messages or previous_response_id

[Cody-learns]

Fixes #3369

Summary

This PR fixes an issue when switching from OpenAIChatModel to OpenAIResponseModel, where the OpenAI Responses API rejects requests with no input and no previous_response_id.

Per maintainer guidance, we detect when both openai_messages and previous_response_id are empty and automatically inject a blank user message.

Changes

• In openai.py, insert a minimal blank EasyInputMessageParam when no input messages exist.
• Add test: test_openai_responses_runs_with_deps_only_and_sends_input, verifying that deps-only runs provide a non-empty input or text field.

[DouweM]

We should clarify that there are instructions, because if not, the request would really be empty and we should error.

Suggested change

	# When there are no input messages and we're not reusing a previous response,
	# the OpenAI API will reject a request without any input. To avoid this provide
	# an explicit empty user message.
	# When there are no input messages and we're not reusing a previous response,
	# the OpenAI API will reject a request without any input,
	# even if there are instructions.
	# To avoid this provide an explicit empty user message.

[DouweM]

• I think we can remove the deps stuff, just having instructions should be enough.
• We should do a real model request and verify the API accepts this payload and behaves correctly; we shouldn't mock the request. If you remove the mock stuff, you can run the test with pytest <path> --record-mode=rewrite and it'll automatically store a pyvcr cassette with the recorded request/response.