fix: return rotated refresh token from agent token refresh endpoint

Previously the refresh endpoint only returned a new accessToken, causing
agents to lose their refreshToken after the first refresh cycle and
forcing a full re-registration every ~2 hours.

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

cameleer3-server-app/src/main/java/com/cameleer3/server/app/dto/AgentRefreshResponse.java

@@ -3,5 +3,5 @@ package com.cameleer3.server.app.dto;
 import io.swagger.v3.oas.annotations.media.Schema;
 import jakarta.validation.constraints.NotNull;
 
-@Schema(description = "Refreshed access token")
-public record AgentRefreshResponse(@NotNull String accessToken) {}
+@Schema(description = "Refreshed access and refresh tokens")
+public record AgentRefreshResponse(@NotNull String accessToken, @NotNull String refreshToken) {}