fix custom provider credentials, edit path, and failure UX

Require an API key or {env:VAR} on client and server, add edit/prefill for
existing custom providers, save auth before config, and surface incomplete
auth plus disconnect after partial failures. Add VS Code parity tests and
drop the unused allProvidersConnected locale key.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/vscode/src/DOCUMENTATION.md

@@ -61,7 +61,7 @@ The webview CSP permits `blob:` only for `worker-src` so shared UI parsers can r
   - Includes session activity snapshot bridge handler used by webview parity routes (`/api/session-activity`).
   - Includes Zen utility model parity handler used by shared notification settings (`/api/zen/models`).
   - Owns managed OpenCode upgrade status and mutation handlers, including capability reporting, upgrade serialization, and process restart after a successful upgrade.
-  - Provider handlers cover source lookup, disconnect (`DELETE /api/provider/:id/auth`), and custom provider upsert (`PUT /api/provider`).
+  - Provider handlers cover source lookup, disconnect (`DELETE /api/provider/:id/auth`), and custom provider upsert (`PUT /api/provider`; create/update OpenAI-compatible config; requires `env` or stored auth; secrets via OpenCode auth API).
 
 - `opencode-upgrade-runtime.ts`
   - Owns managed-versus-external capability decisions, latest-version checks, serialized OpenCode self-upgrades, and restart-after-upgrade behavior.