feat(channel-monitor): preserve upstream error body

Monitor:
- callProvider now returns both textPath-extracted text and raw body;
  runCheckForModel uses rawBody on non-2xx so history.message stops being
  "upstream HTTP 503: " with empty body (gjson textPath produces "" for
  error responses like {"error":{"message":"No available accounts..."}})
- truncateForErrorBody collapses whitespace then caps at 300 bytes
  (monitorErrorBodySnippetMaxBytes); final truncateMessage still enforces
  the 500-byte DB column cap

Frontend:
- MonitorFormDialog: primary_model input text color and ModelTagInput tags
  now both track form.provider (via new getPlatformTextClass + existing
  getPlatformTagClass with platform prop).

(cherry-picked from 1d3b0418; dropped gateway_handler logging改动，不在本 PR 范围)

frontend/src/components/admin/channel/types.ts

@@ -187,3 +187,14 @@ export function getPlatformTagClass(platform: string): string {
     default: return 'bg-gray-100 text-gray-700 dark:bg-gray-900/30 dark:text-gray-400'
   }
 }
+
+/** 平台对应的模型文字色（仅 text-*，用于 input/text 场景）— 与 getPlatformTagClass 同色系 */
+export function getPlatformTextClass(platform: string): string {
+  switch (platform) {
+    case 'anthropic': return 'text-orange-700 dark:text-orange-400'
+    case 'openai': return 'text-emerald-700 dark:text-emerald-400'
+    case 'gemini': return 'text-blue-700 dark:text-blue-400'
+    case 'antigravity': return 'text-purple-700 dark:text-purple-400'
+    default: return ''
+  }
+}