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.

tests/test_config_flow.py

@@ -154,4 +154,25 @@ def test_no_hardcoded_domain_or_db_filename_in_new_modules() -> None:
                     )
                 assert node.value != "nestquest.db", (
                     f"Found hard-coded database filename in {py_file.name}:{node.lineno}"
-                )
+                )
+
+async def test_flow_persists_flow_user_as_initial_admin(hass, make_flow) -> None:
+    """The HA user completing the flow lands in entry data as the
+    initial admin copy: entry.context is not persisted by real HA, so
+    the durable record must carry the owner."""
+    from custom_components.nestquest.const import CONF_ADMIN_USER_IDS
+
+    flow = make_flow(hass)
+    flow.context = {"user_id": "ha-owner"}
+    result = await flow.async_step_user({})
+    assert result["type"] == "create_entry"
+    assert result["data"] == {CONF_ADMIN_USER_IDS: ["ha-owner"]}
+
+
+async def test_flow_without_context_user_keeps_data_untouched(
+    hass, make_flow
+) -> None:
+    flow = make_flow(hass)
+    result = await flow.async_step_user({"horizon_days": 7})
+    assert result["type"] == "create_entry"
+    assert result["data"] == {"horizon_days": 7}