Fix invalid input_text type when sending model messages back to OpenAI Responses API

[felixarntz]

Fixes #182

Supersedes #183

I've added a basic integration test for multi-turn messages, for the other providers too, just to make sure it works and is covered.

You can verify that this fixes the bug by temporarily changing the output_text string in my changed line back to input_text - this would lead to the original behavior before this PR, and you'll get the exact exception that #182 mentions.

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[JasonTheAdams]

Great catch! I'm feeling better and better about these integration tests.

[jmarx]

@felixarntz Thanks! I actually have not had a chance to test your version. But I assume it works :)