Handle signature_delta streaming event for Anthropic thinking blocks

The Anthropic streaming API sends thinking block signatures via a
signature_delta event, not in content_block_start or content_block_stop.
The parser was ignoring this event type, falling back to the empty
placeholder signature from content_block_start, causing "Invalid
signature in thinking block" errors on multi-turn conversations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/llm/src/providers/anthropic.rs

@@ -800,6 +800,19 @@ impl StreamAccumulator {
                     delta: thinking.to_string(),
                 }]
             }
+            "signature_delta" => {
+                let signature = delta
+                    .and_then(|d| d.get("signature"))
+                    .and_then(serde_json::Value::as_str)
+                    .map(String::from);
+                if let Some(ContentBlockKind::Thinking {
+                    signature: ref mut sig,
+                }) = self.current_block
+                {
+                    *sig = signature;
+                }
+                vec![]
+            }
             _ => vec![],
         }
     }