Review fixes: persisted flow-user admin seed and real cv.multi_select

Review rounds 1-2 (Codex gpt-5.6-terra) found two HIGHs against real HA:
- entry.context is not a durable record — the config flow now persists
  the completing HA user into entry data (CONF_ADMIN_USER_IDS) and the
  seed precedence becomes: options copy (the current list) first, then
  the config flow's entry-data copy, then the context user as a last
  resort. Config-flow tests cover the persisted user and the
  context-free path; a seed-precedence test covers options > data.
- the hand-rolled closure could not serialize as an HA multi-select.
  The harness now mocks homeassistant.helpers.config_validation with a
  faithful multi_select, and the flow uses cv.multi_select exactly as
  real HA code does. Suite: 904 passed.

custom_components/nestquest/options_flow.py

@@ -7,6 +7,7 @@ from typing import Any
 import voluptuous as vol
 from homeassistant import config_entries
 from homeassistant.data_entry_flow import FlowResult
+from homeassistant.helpers import config_validation as cv
 
 from .const import (
     CONF_ADMIN_USER_IDS,
@@ -31,29 +32,6 @@ def _parse_time(value: str) -> bool:
     return _TIME_PATTERN.fullmatch(value) is not None
 
 
-def _multi_select(choices: dict[str, str]):
-    """A voluptuous validator for a multi-select of ``choices``.
-
-    The mock-only test harness cannot import HA's config-validation
-    helpers (they pull in real HA), so this mirrors the shape of
-    ``homeassistant.helpers.config_validation.multi_select``: accepts a
-    list of keys from ``choices``, returns the list unchanged, rejects
-    anything else (None becomes the empty selection).
-    """
-
-    def _validate(value):
-        if value is None:
-            return []
-        if not isinstance(value, list):
-            raise vol.Invalid("expected a list of selections")
-        for item in value:
-            if item not in choices:
-                raise vol.Invalid(f"{item!r} is not a valid choice")
-        return list(value)
-
-    return _validate
-
-
 def _async_validate(user_input: dict[str, Any]) -> dict[str, str]:
     """Validate user input and return field-level errors instead of raising."""
     errors: dict[str, str] = {}
@@ -106,11 +84,13 @@ def _build_schema(
         ): int,
     }
     if admin_choices is not None:
+        # HA's supported multi-select validator: the frontend can
+        # serialize the field as a real picker of the given choices.
         fields[
             vol.Optional(
                 CONF_ADMIN_USER_IDS, default=list(admin_default or [])
             )
-        ] = _multi_select(admin_choices)
+        ] = cv.multi_select(admin_choices)
     return vol.Schema(fields)