refactor(BA-6003): use BackendAIModelValidationFailed.errors() accessor

Switch every caller that iterates ``BackendAIModelValidationFailed``'s
per-field error list from the verbose
``(e.extra_data or {}).get("errors", [])`` to the new ``e.errors()``
classmethod (mirrors ``pydantic.ValidationError.errors()``):

* 7 ``_build_dto`` CLI helpers in ``client/cli/v2/admin/*`` and
  ``client/cli/v2/model_card/commands.py``.
* The ``pydantic_params_api_handler`` decorator in
  ``manager/api/utils.py``.

Also drop a duplicate ``ModelDefinitionValidationError`` class that
slipped back in during a rebase.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/client/cli/v2/admin/deployment_revision_preset.py

@@ -25,7 +25,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/client/cli/v2/admin/model_card.py

@@ -25,7 +25,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/client/cli/v2/admin/resource_policy.py

@@ -54,7 +54,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/client/cli/v2/admin/resource_preset.py

@@ -26,7 +26,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/client/cli/v2/admin/runtime_variant.py

@@ -25,7 +25,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/client/cli/v2/admin/runtime_variant_preset.py

@@ -25,7 +25,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/client/cli/v2/model_card/commands.py

@@ -25,7 +25,7 @@ def _build_dto(dto_cls: type, data: dict[str, Any]) -> Any:
         return dto_cls.model_validate(data)
     except BackendAIModelValidationFailed as e:
         click.echo("Validation error:", err=True)
-        for err in (e.extra_data or {}).get("errors", []):
+        for err in e.errors():
             field = ".".join(str(loc) for loc in err["loc"])
             click.echo(f"  {field}: {err['msg']}", err=True)
         sys.exit(1)

src/ai/backend/common/exception.py

@@ -493,24 +493,6 @@ def error_code(self) -> ErrorCode:
         )
 
 
-class ModelDefinitionValidationError(BackendAIError, web.HTTPBadRequest):
-    """Raised by :class:`ai.backend.common.config.ModelDefinition` when
-    its ``model_validate`` call fails. Lives in ``common`` so the model
-    itself (also in ``common``) can construct it via
-    :meth:`BackendAIModel.build_validation_error`, with no caller-side
-    re-wrap needed."""
-
-    error_type = "https://api.backend.ai/probs/model-definition-validation-failed"
-    error_title = "Model definition validation failed."
-
-    def error_code(self) -> ErrorCode:
-        return ErrorCode(
-            domain=ErrorDomain.MODEL_SERVICE,
-            operation=ErrorOperation.ACCESS,
-            error_detail=ErrorDetail.INVALID_PARAMETERS,
-        )
-
-
 class DeprecatedAPI(BackendAIError, web.HTTPBadRequest):
     error_type = "https://api.backend.ai/probs/deprecated"
     error_title = "This API is deprecated."

src/ai/backend/manager/api/utils.py

@@ -224,8 +224,7 @@ async def wrapped(
             except (json.decoder.JSONDecodeError, yaml.YAMLError, yaml.MarkedYAMLError) as e:
                 raise InvalidAPIParameters("Malformed body") from e
             except BackendAIModelValidationFailed as ex:
-                errors = (ex.extra_data or {}).get("errors") or []
-                first_error = errors[0]
+                first_error = ex.errors()[0]
                 # Format the first validation error as the message
                 # The client may refer extra_data to access the full validation errors.
                 metadata = {