Sanitize Responses message content for assistant history and images

[Futureppo]

Summary

• Normalize assistant message history content parts from output_text to upstream-compatible input_text.
• Avoid adding an explicit content: null field when a message item has no content key.
• Rewrite Anthropic-style message image blocks (type: image with source) to Responses-compatible input_image parts before forwarding upstream.
• Add regression coverage for assistant output text, missing content, and Anthropic-style image content.

Testing

• go test ./internal/openai ./internal/anthropic ./internal/server
• go test ./...

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Codex Task

[gemini-code-assist]

Code Review

This pull request introduces message content sanitization for OpenAI-compatible responses, specifically converting assistant output_text to input_text and rewriting Anthropic-style image content (both URL and base64 sources) into standard input_image payloads. Feedback on the changes suggests validating that both data and media_type are non-empty when processing base64 image sources to prevent generating invalid data URLs.

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[gemini-code-assist]

When source type is base64, if data or media_type is empty or missing, url will be constructed as an invalid data URL (e.g., "data:;base64,"), which is not empty and thus bypasses the url == "" check. This will result in sending an invalid image payload upstream. We should validate that both data and media_type are non-empty before constructing the data URL.

Suggested change

	case "base64":
	url = "data:" + String(source, "media_type", "") + ";base64," + String(source, "data", "")
	case "base64":
	data := String(source, "data", "")
	mediaType := String(source, "media_type", "")
	if data == "" || mediaType == "" {
	return nil
	}
	url = "data:" + mediaType + ";base64," + data