fix: return synthetic empty text on Anthropic empty content response

Anthropic returns an empty content array (stop_reason=end_turn) after
side-effect-only tool calls like react or skip — the model has nothing
further to say but the response is valid.

Previously this was treated as an error, exhausting retries and falling
through to the next model in the fallback chain, which then failed for
unrelated reasons (e.g. deepseek-reasoner expecting reasoning_content).

Return a synthetic empty Text response instead so the agentic loop
terminates gracefully.

Author: Marenz

src/llm/model.rs

@@ -1253,15 +1253,21 @@ fn parse_anthropic_response(
         }
     }
 
-    let choice = OneOrMany::many(assistant_content).map_err(|_| {
+    let choice = OneOrMany::many(assistant_content).unwrap_or_else(|_| {
+        // Anthropic returns an empty content array when stop_reason is end_turn
+        // and the model has nothing further to say (e.g. after a side-effect-only
+        // tool call like react/skip). Treat this as a clean empty response rather
+        // than an error so the agentic loop terminates gracefully.
+        let stop_reason = body["stop_reason"].as_str().unwrap_or("unknown");
         tracing::debug!(
-            stop_reason = body["stop_reason"].as_str().unwrap_or("unknown"),
+            stop_reason,
             content_blocks = content_blocks.len(),
-            raw_content = %body["content"],
-            "empty assistant_content after parsing Anthropic response"
+            "empty assistant_content from Anthropic — returning synthetic empty text"
         );
-        CompletionError::ResponseError("empty response from Anthropic".into())
-    })?;
+        OneOrMany::one(AssistantContent::Text(Text {
+            text: String::new(),
+        }))
+    });
 
     let input_tokens = body["usage"]["input_tokens"].as_u64().unwrap_or(0);
     let output_tokens = body["usage"]["output_tokens"].as_u64().unwrap_or(0);