[Bug] Missing provider credential returns HTTP 500 instead of a structured 4xx (breaks Workspace widget validation) #7628

Describe the bug
When a provider credential is not configured, REST API calls to that provider's endpoints return HTTP 500 with the underlying provider message embedded in prose. A missing credential is an expected configuration state (the standard install leaves most provider keys unset), not an internal server error.

This has a concrete downstream cost: OpenBB Workspace's "Connect backend → Validate widgets" flow treats the 500 as a broken backend and refuses to add an otherwise healthy self-hosted openbb-platform-api backend, because widgets for unconfigured providers can never validate.

To Reproduce

1. OpenBB Platform 4.7.2 with openbb-us-eia installed and eia_api_key unset.
2. openbb-api --host 127.0.0.1 --port 6900
3. curl -i "http://127.0.0.1:6900/api/v1/commodity/short_term_energy_outlook?provider=eia"

Response (HTTP 500):

{"detail": "Error fetching data from the EIA API -> API_KEY_MISSING -> No api_key was supplied.  Please register for one at https://www.eia.gov/opendata/register.php"}

Expected behavior
A structured non-5xx response that machines can classify, e.g. HTTP 400 or 424 with:

{"detail": {"code": "missing_credentials", "provider": "eia", "credential": "eia_api_key"}}

That would let clients (including the Workspace widget validator) distinguish "unconfigured provider" from "server fault", and would let openbb-platform-api (which runs in-process and can see credentials) annotate or exclude affected widgets in widgets.json.

Desktop:

• OS: Debian 12 (python:3.12-slim container, linux/amd64)
• Python version: 3.12
• OpenBB Platform version: openbb 4.7.2, openbb-platform-api 1.3.6