feat: Add Chat Generator supporting OpenAI Responses API

[Amnah199]

Related Issues

• fixes Add support for OpenAI's Responses API #9422

Proposed Changes:

• Add a chat generator that uses openai's responses endpoint.
• The tools param in init and run methods allows openai and MCP tools besides haystack Tools.
• Structured outputs can be enabled by passing text_format in generation_kwargs.
• Reasoning can be enabled by setting reasoning params in generation_kwargs.
• There are other features in Responses API like background requests etc, which are not supported in this iteration.
• With every ToolCall sent back, the API also expects the reasoning id even if the reasoning object was empty.

How did you test it?

• Add new unit tests to test features.
• Add new integration tests.

Notes for the reviewer

Using OpenAI tools and MCP tools doesn’t behave like standard function calls.

• Tool calls are usually identified by type="function_call". However, for OpenAI/MCP tools, the type value varies depending on the tool being used — for example, type="web_search_call".
• Currently, we only return standard ResponseFunctionToolCall objects. As a result, tool calls from OpenAI/MCP tools won’t appear in the ChatMessage, so the user won’t see them.
• This is fine, since these tool calls are handled internally by OpenAI to generate the final response. The user already receives the output.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I documented my code
• I ran pre-commit hooks and fixed any issue

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 91.566%

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Totals
Change from base Build 19102178311:	-0.7%
Covered Lines:	13668
Relevant Lines:	14927

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💛 - Coveralls

[sjrl]

@Amnah199 can we make sure to update the docs files in docs/pydoc/config and docs/pydoc/config_docusaurus?

[sjrl]

Looks good!

[Amnah199]

@sjrl This value is True by default in Responses. For consistency with other chat generators I kept it false.