GET/PATCH /api/providers/config, subset POST /refresh, and
GET /api/providers/:id/diagnostic (JSON { diagnostic }, §6.4). PATCH order
is validate→save→reload→clear; a malformed body or invalid merged config
returns 422 without writing, and a save failure returns 500 without
reloading (no file/registry divergence). Web client + types extended.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>