Validate every model-specific task after an AI credential change

Reusable investigation method: a working replacement AI credential does not establish access to every model used by an application. Inventory task-specific model constants, then validate each model/task path. Separate billing/quota failures from model-access rejection, and locate the first occurrence on both sides of the suspected change instead of assuming a release caused it. When per-item exceptions are converted to unknown results but the enclosing task reports success, use provider-completion evidence plus per-item failure counts and unique workflows; task success alone is not a valid denominator. Generalized from a read-only Shield release investigation on 2026-09-09; provenance: source-level model selection and exception handling, with aggregate runtime evidence used to develop the method. No current configuration, customer data, or operational status is included.