Closes #27290

The model_config_id validation in coderd/exp_chats.go returned a single ambiguous message, "Invalid model_config_id: model config not found or disabled.", for both the not-found and disabled-model cases.

Split the shared chatModelConfigUnavailableModelNotFoundOrDisabled reason into chatModelConfigUnavailableModelNotFound and chatModelConfigUnavailableModelDisabled so each maps to a distinct, actionable message:

• not found: "Invalid model_config_id: model config not found."
• disabled: "Invalid model_config_id: this model is disabled. Enable it or choose another model."

The provider-disabled and credentials-missing reasons already had their own distinct messages and are unchanged.

Tests: updated the existing disabled-model assertion and added a not-found case under TestPostChats.

Note: the (guarded, should-never-happen) branch for an enabled config with an unset provider FK now maps to the "not found" message rather than introducing a fourth reason for an unreachable state. Happy to add a dedicated message if a reviewer prefers.

This PR was generated by Coder Agents on behalf of @stirby.