Fix 'list' object has no attribute 'strip' error

Handle multimodal messages (images) where content is a list instead of string.
Skip non-string content in LLMLingua and code-aware compression transforms.

headroom/transforms/code_compressor.py

@@ -1201,7 +1201,8 @@ class CodeAwareCompressor(Transform):
         for message in messages:
             content = message.get("content", "")
 
-            if not content:
+            # Skip empty or non-string content (multimodal messages with images)
+            if not content or not isinstance(content, str):
                 transformed_messages.append(message)
                 continue
 
@@ -1266,7 +1267,8 @@ class CodeAwareCompressor(Transform):
 
         for message in messages:
             content = message.get("content", "")
-            if content:
+            # Only check string content (skip multimodal)
+            if content and isinstance(content, str):
                 detection = detect_content_type(content)
                 if detection.content_type == ContentType.SOURCE_CODE:
                     return True

headroom/transforms/llmlingua_compressor.py

@@ -430,6 +430,11 @@ class LLMLinguaCompressor(Transform):
             role = message.get("role", "")
             content = message.get("content", "")
 
+            # Skip non-string content (multimodal messages with images)
+            if not isinstance(content, str):
+                transformed_messages.append(message)
+                continue
+
             # Compress tool results (highest value compression)
             if role == "tool" and content:
                 result = self.compress(content, context=context, content_type="json")