Add summary/title to apply_file_diffs tool and propagate is_error on tool results

The apply_file_diffs tool now includes a required "summary" field so the
model provides a brief description of edits. This surfaces as the title
on the diff panel instead of showing only +/- line counts.

Also propagates is_error from tool result content back to the model so it
can distinguish failed tool calls from successful ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ai/src/agent/action/convert.rs

@@ -126,7 +126,7 @@ impl From<api::message::tool_call::ApplyFileDiffs> for AIAgentActionType {
                 .chain(new_file_edits)
                 .chain(file_deletes)
                 .collect(),
-            title: Some(value.summary),
+            title: value.summary.none_if_default(),
         }
     }
 }