(bug) A self_managed connection carrying a slug returns HTTP 500 instead of 422

[mmabrouk]

When I send an agent run whose connection is {"mode": "self_managed", "slug": "..."}, the request fails with HTTP 500. The validator that rejects the combination (Connection._reject_slug_for_self_managed in sdks/python/agenta/sdk/agents/connections/models.py) raises during request handling and surfaces as an internal error, while the sibling capability rejection for an unsupported provider correctly returns 422.

Steps to reproduce:

1. POST an agent run with connection: {"mode": "self_managed", "slug": "anything"}.
2. Response: 500 with the validator's message, instead of a 422 validation response.

A client input error should never surface as a 500. Seen on staging (v0.107.0) during release QA, 2026-08-01; the validator predates this release (landed 2026-06-24).

[linear-code]

AGE-4051

[dosubot]

The root cause is that the _reject_slug_for_self_managed validator uses Pydantic's built-in @model_validator, which raises a ValidationError. FastAPI only auto-converts ValidationError to 422 when it occurs during request body parsing. Since the Connection model is validated inside the agent invoke handler/service layer (not as a top-level request model), the ValidationError propagates as an unhandled exception and gets caught by the generic exception interceptor, which returns 500 [1].

The capability check (provider 'custom' is not supported by harness 'pi_core') correctly returns 422 because it raises a domain-specific exception (like UnsupportedDeploymentError) that is explicitly mapped to 422 — it doesn't rely on Pydantic's validation machinery at all [2].

The cleanest fix would be to catch the ValidationError from the Connection model instantiation and re-raise it as an UnprocessableContentException [3], which the API already provides for explicit 422 responses. Alternatively, the @model_validator could be replaced with a pre-validation check that raises a domain-specific exception, matching the pattern used by capability validation.

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