Model Error Override System

• A new config key tengu-model-error-overrides is read from server-delivered configuration. It maps model identifiers to objects with optional block (error message text) and pickerHint (hint shown in the model picker) fields.
• When a matching override is present and its block string is non-empty, the override text replaces the default error display.
• pickerHint can provide a separate, shorter hint for use in the model selection UI. If absent, the block text is reused.
• If any error occurs reading the override config, the system logs model-error-overrides picker hint failed: or tengu-model-error-overrides block check failed: and falls back to default behavior.

This is a server-controlled capability with no user-facing knob; the overrides are delivered via the existing feature-flag config mechanism and take effect automatically when present.