Tool content validation

[willccbb]

Description

Tightens the content validation in ToolEnv.call_tool to ensure that tool return values are only preserved as lists if they conform to the text or image_url content part format. Any other list types or invalid content part structures are now cast to a string.

This change aligns tool output handling with expected message formats, allowing for structured text and image_url content while ensuring other unexpected list formats are serialized.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

While the OpenAI SDK (v2.16.0) officially supports only str | Iterable[ChatCompletionContentPartTextParam] for ChatCompletionToolMessageParam content, this implementation includes image_url support for tool returns to be forward-compatible with providers that may accept this format, and to align with potential practical use cases as suggested by the task.

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Note

Low Risk
Small, well-tested validation change limited to how tool return values are serialized into tool messages; main risk is behavior change for tools that previously returned arbitrary lists.

Overview
Tightens tool output handling in ToolEnv.call_tool. Tool return values are now preserved as a list only when they validate as structured content parts with type: "text" or type: "image_url"; all other list-shaped returns are coerced to str.

Adds is_valid_tool_content_parts (with a shared VALID_TOOL_CONTENT_PART_TYPES) and expands tests to cover valid/mixed content parts as well as invalid list shapes that should be stringified.

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