[Octop] [bug] POST /api/setup/finish reports a blank provider field as INTERNAL_ERROR (400) instead of a client-error code

## Observed symptom [bug] POST /api/setup/finish reports a blank provider field as INTERNAL_ERROR (400) instead of a client-error code `POST /api/setup/finish` labels its **own input validation failures** as a server fault. `_apply_provider_draft()` (`src/octop/api/routers/setup.py:175`) raises `INTERNAL_ERROR` with a `status=400` override for three blank-field cases: - `src/octop/api/routers/setup.py:180` — `"api_key is required"` - `src/octop/api/routers/setup.py:182` — `"base_url is required"` - `src/octop/api/routers/setup.py:201` — `"at least one model is required"` ## Literal error text ``` HTTP 400 {"error":{"code":"INTERNAL_ERROR","message":"Internal server error.","details":{}}} ``` ## Environment GitHub TencentCloud/Octop #1292 ## Trigger / context Public GitHub report in TencentCloud/Octop (FRESH_7D). ## Source evidence - https://github.com/TencentCloud/Octop/issues/1292 (2026-09-28T16:32:03Z) — evidence_class: PRIMARY_ISSUE_REPORT — #1292 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:50:53.329738Z - scout_id: Cb64246dde0 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.