GitLab

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)