MessageData is lossy for multimodal UserMessage content

[jasperblues]

MessageData (embabel-chat-store/src/main/kotlin/com/embabel/chat/store/model/StoredMessage.kt) stores message content as a single content: String. Round-tripping a multimodal UserMessage through the store silently discards every non-text part.

The loss happens at both ends:

• MessageData.from(message) captures only message.content. For a BaseMessage that is textContent — the concatenation of TextParts only, with ImagePart, DocumentPart and MediaPart dropped.
• MessageData.toMessage() rebuilds UserMessage(content = content, ...), whose convenience constructor wraps the string into listOf(TextPart(content)).

So a rehydrated conversation is text-only even when the original message had attachments. In embabel-agent, BaseMessage explicitly models this content (parts: List<ContentPart>, with imageParts, documentParts, mediaParts, and isMultimodal accessors), so the capability exists upstream and is lost purely at the persistence boundary.

Impact: an application that reloads a conversation and re-sends it cannot rely on the store for attachments. The workaround is to keep the original in-memory message alongside the persisted one and substitute it back before invoking the LLM — which defeats the point of having a conversation store, and only works while the original is still in memory.

Question: is multimodal persistence intended to be in scope for the chat store? If so, could MessageData preserve the parts — either inline or as durable references (URI + mime type + size) with the bytes in a separate store?

Note the same class of gap exists on the assistant side: AssistantMessage carries assets: List<Asset> and implements AssetView, but MessageData has no assets field either. Related: embabel/embabel-agent#1821.