Use ValueError on 'too many columns'; drop stale enum-guard test

Two small fixes tied together:

- `_validate_import_line_or_throw` raised AttributeError on the
  too-many-columns branch; `create_user_from_line` uses ValueError for
  the same condition. Copilot review flagged the inconsistency in #1811
  since callers using `_validate_import_line_or_throw` directly have to
  catch a different exception than they would from
  `create_user_from_line` for the same malformed input. Use ValueError
  throughout for input-shape violations.

- `test_set_values_rejects_invalid_auth_setting` asserted _set_values
  raises ValueError on an unknown auth string. That's exactly the
  behavior b088d3d removed (server-parse paths must accept unknown
  future auth values). b088d3d added the positive test
  (`test_from_xml_accepts_unknown_auth_setting` in test_user.py) but
  forgot to delete the contradictory one. Drop it.

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

Author: jacalata

tableauserverclient/models/user_item.py

@@ -539,7 +539,7 @@ def _validate_import_line_or_throw(incoming, logger) -> None:
 
             line = list(map(str.strip, incoming.split(",")))
             if len(line) > UserItem.CSVImport.COLUMN_COUNT:
-                raise AttributeError("Too many attributes in line")
+                raise ValueError("Too many attributes for user import")
             username = line[UserItem.CSVImport.ColumnType.USERNAME.value]
             logger.debug(f"> details - {username}")
             UserItem.validate_username_or_throw(username)

test/test_user_model.py

@@ -183,11 +183,3 @@ def test_create_user_with_lowercase_auth_accepted() -> None:
     user = TSC.UserItem.CSVImport.create_user_from_line("username, pword, fname, creator, none, yes, email, saml")
     assert user is not None
     assert user.auth_setting == "SAML"
-
-
-def test_set_values_rejects_invalid_auth_setting() -> None:
-    # _set_values must route auth_setting through the @property_is_enum guard.
-    # Attempting to set an invalid auth string must raise, not silently succeed.
-    user = TSC.UserItem("someone")
-    with pytest.raises(ValueError):
-        user._set_values(None, None, None, None, None, None, None, "NotAnAuthType", None, None, None, None)