Simplify role handling in prepare_multimodal_messages

[albertvillanova]

Simplify role handling in prepare_multimodal_messages.

Follow-up to:

• Fix prepare_multimodal_messages not normalizing empty string content for assistant/tool roles #5496

This PR simplifies and consolidates the logic in the prepare_multimodal_messages function for handling different message roles. The main change is that the preparation of message content for the assistant, system, and tool roles is now handled together, reducing code duplication and improving maintainability.

Changes

Refactoring and logic simplification:

• Consolidated the handling of assistant, system, and tool message roles into a single code block, reducing repeated code for preparing message content as a list of text entries.
• Removed the special-casing for the system role at the top of the function, as it is now handled together with the other roles.

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Note

Low Risk
Low risk refactor that consolidates per-role branching without changing the public API; main risk is subtle differences in how string content is wrapped for non-user roles.

Overview
Simplifies prepare_multimodal_messages by consolidating the system, assistant, and tool role handling into a single branch that wraps string content into structured [{"type": "text", ...}] blocks.

Keeps the user-role logic for injecting image placeholders before the first user message, while reducing duplicated role-specific code paths.

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[qgallouedec]

looks good, I'm sure it's on your radar, but tool could contain images now

[albertvillanova]

tool could contain images now

Yes, @qgallouedec, I think that tool images are already in-lined (not using image placeholders). A somehow related enhancement is addressed in:

• Support messages with images in prepare_multimodal_messages #5474