fix: validate content block type in Anthropic assistant blocks (#4060)

_assistant_blocks appends dict items from content lists directly
without checking for the required 'type' field. A block like
{'text': 'hi'} reaches the Anthropic payload without a 'type',
causing a 400 rejection.

Add the same missing-type check that _convert_user_content already has,
so bare dicts in assistant content lists are coerced to text blocks
instead of triggering API validation errors.

Co-authored-by: nanobot-issues <issues@nanobot.dev>

nanobot/providers/anthropic_provider.py

@@ -275,7 +275,16 @@ class AnthropicProvider(LLMProvider):
             blocks.append({"type": "text", "text": content})
         elif isinstance(content, list):
             for item in content:
-                blocks.append(item if isinstance(item, dict) else {"type": "text", "text": str(item)})
+                if isinstance(item, dict):
+                    if not item.get("type"):
+                        # Anthropic requires every content block to declare a "type".
+                        # A tool that returned a bare dict lands here; coerce it to
+                        # a text block instead of emitting one that the API rejects.
+                        blocks.append({"type": "text", "text": str(item)})
+                    else:
+                        blocks.append(item)
+                else:
+                    blocks.append({"type": "text", "text": str(item)})
 
         for tc in msg.get("tool_calls") or []:
             if not isinstance(tc, dict):