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/test/java/com/cameleer3/server/app/security/JwtRefreshIT.java

@@ -79,6 +79,8 @@ class JwtRefreshIT extends AbstractPostgresIT {
 
         JsonNode body = objectMapper.readTree(response.getBody());
         assertThat(body.get("accessToken").asText()).isNotEmpty();
+        assertThat(body.get("refreshToken").asText()).isNotEmpty();
+        assertThat(body.get("refreshToken").asText()).isNotEqualTo(refreshToken);
     }
 
     @Test