fix(core): handle empty content in Anthropic response gracefully (#9345)

Fixes Datadog issue a47619fa-d5b8-11f0-9fd7-da7ad0900000

Handle empty content in Anthropic responses gracefully by replacing RuntimeError with LLMServerError. Now logs detailed debugging information (response ID, model, stop_reason) and returns a user-friendly error instead of crashing.

🐾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/llm_api/anthropic_client.py

@@ -1182,7 +1182,23 @@ class AnthropicClient(LLMClientBase):
                     redacted_reasoning_content = content_part.data
 
         else:
-            raise RuntimeError("Unexpected empty content in response")
+            # Log the full response for debugging
+            logger.error(
+                "[Anthropic] Received response with empty content. Response ID: %s, Model: %s, Stop reason: %s, Full response: %s",
+                response.id,
+                response.model,
+                response.stop_reason,
+                json.dumps(response_data),
+            )
+            raise LLMServerError(
+                message=f"LLM provider returned empty content in response (ID: {response.id}, model: {response.model}, stop_reason: {response.stop_reason})",
+                code=ErrorCode.INTERNAL_SERVER_ERROR,
+                details={
+                    "response_id": response.id,
+                    "model": response.model,
+                    "stop_reason": response.stop_reason,
+                },
+            )
 
         assert response.role == "assistant"
         choice = Choice(