fix: show retry attempt count when aborting during retry

When aborting a retry attempt, surface the retry-aware abort message
in the assistant output and tool results instead of a generic "Aborted".

- Set errorMessage for aborted streaming messages
- Render abort message without forcing a leading newline when no content

packages/coding-agent/src/core/agent-session.ts

@@ -435,6 +435,11 @@ export class AgentSession {
 		return this.agent.state.isStreaming;
 	}
 
+	/** Current retry attempt (0 if not retrying) */
+	get retryAttempt(): number {
+		return this._retryAttempt;
+	}
+
 	/**
 	 * Get the names of currently active tools.
 	 * Returns the names of tools currently set on the agent.