fix(translator): don't drop image-only user messages in prepareClaudeRequest

hasValidContent() only treated text/tool_use/tool_result blocks as valid
content, so a user message containing only an image block was filtered
out as empty. When it was the only non-system message, this left an
empty messages array and Anthropic rejected the request.

open-sse/translator/formats/claude.js

@@ -16,7 +16,9 @@ export function hasValidContent(msg) {
     return msg.content.some(block =>
       (block.type === CLAUDE_BLOCK.TEXT && block.text?.trim()) ||
       block.type === CLAUDE_BLOCK.TOOL_USE ||
-      block.type === CLAUDE_BLOCK.TOOL_RESULT
+      block.type === CLAUDE_BLOCK.TOOL_RESULT ||
+      block.type === CLAUDE_BLOCK.IMAGE ||
+      block.type === CLAUDE_BLOCK.DOCUMENT
     );
   }
   return false;