fix: do not mutate Tool schemas (and handle empty tools) in OpenAIResponsesChatGenerator

[TimurRakhmatullin86]

Related Issues

None — two self-contained bugs found in OpenAIResponsesChatGenerator tool handling.

Proposed Changes

1. Do not mutate the user's Tool schema in place.
In _prepare_api_call, tool definitions were built like:

function_spec = {**t.tool_spec}
function_spec["parameters"]["additionalProperties"] = False

Tool.tool_spec returns {"name": ..., "description": ..., "parameters": self.parameters} — parameters by reference — and {**t.tool_spec} only shallow-copies the outer dict. So additionalProperties: False was written permanently into the caller's live Tool.parameters. Consequences: the same Tool passed to another generator (e.g. OpenAIChatGenerator with tools_strict=False, which intentionally leaves schemas open) is contaminated, and tool.to_dict() / a serialize→load→serialize round trip is no longer stable. Fixed by building the definition on a copied parameters dict: {**function_spec["parameters"], "additionalProperties": False}.

2. Guard _warm_up_tools against an empty tools list.
is_openai_tool = isinstance(self.tools, list) and isinstance(self.tools[0], dict) raised IndexError for tools=[] (an ordinary value when tools are built programmatically). Every sibling check already guards emptiness; this one now does too via bool(self.tools).

How did you test it?

Unit tests in test/components/generators/chat/test_openai_responses.py:

• test_prepare_api_call_does_not_mutate_tool_parameters — asserts the original Tool.parameters is untouched while the API payload still carries additionalProperties=False.
• test_warm_up_with_empty_tools_list_does_not_raise — warms up with tools=[] without error.

Both fail against the current code (AssertionError / IndexError respectively) and pass with the fix. Full test_openai_responses.py unit suite passes; ruff check/ruff format --check clean.

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