Fix Anthropic image content normalization

[Futureppo]

Motivation

• Fix handling of Anthropic-style type: "image" content in /v1/messages so it no longer triggers unsupported-type errors during OpenAI chat preparation.
• Ensure Anthropic image blocks (URL and base64 sources) are normalized into OpenAI-compatible image_url parts and client-provided metadata is stripped/recorded as a change.

Description

• Added a new case "image" branch in prepareMessageContent to accept Anthropic image blocks and map them to image_url parts (file: internal/openai/chat_prepare.go).
• Implemented prepareAnthropicImage to validate source objects, support url and base64 sources (constructing data:<media_type>;base64,<data>), and record the rewrite with recordUnknownObjectFields and change.
• Updated the chat preparation regression test TestPrepareChatNormalizesMessagesAndToolHistory to include an Anthropic type: "image" block and assert it is normalized and tracked (file: internal/openai/chat_prepare_test.go).

Testing

• Ran the full test suite with go test ./... and all tests succeeded.

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Codex Task

[gemini-code-assist]

Code Review

This pull request introduces support for normalizing Anthropic-style image blocks (with both URL and base64 sources) into OpenAI-compatible image_url objects, along with corresponding unit tests. The feedback suggests trimming whitespace from the base64 media_type and data fields before validation and URL construction to improve robustness against leading or trailing whitespace.

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[gemini-code-assist]

When constructing a data URL for base64-encoded images, any leading or trailing whitespace (such as newlines, which are very common in base64 payloads) in media_type or data can result in an invalid data URL. Trimming these values before validation and URL construction ensures robustness and compatibility with various client inputs.

Suggested change

	mediaType, _ := source["media_type"].(string)
	data, _ := source["data"].(string)
	if strings.TrimSpace(mediaType) == "" || strings.TrimSpace(data) == "" {
	return nil, fmt.Errorf("%s.source media_type and data are required for base64 images", path)
	}
	url = "data:" + mediaType + ";base64," + data
	mediaType, _ := source["media_type"].(string)
	data, _ := source["data"].(string)
	mediaType = strings.TrimSpace(mediaType)
	data = strings.TrimSpace(data)
	if mediaType == "" || data == "" {
	return nil, fmt.Errorf("%s.source media_type and data are required for base64 images", path)
	}
	url = "data:" + mediaType + ";base64," + data