fix(config_flow): D3 — await the discovery-dedup coroutine (t37)

`ConfigFlow._async_handle_discovery_without_unique_id` is an `async def` in core
(verified: `inspect.iscoroutinefunction` is True) and core awaits it. t32 called
it without `await`, so the coroutine body never ran: no `already_configured`
abort when the handler already had entries, no
`DEFAULT_DISCOVERY_UNIQUE_ID` claim, no `already_in_progress` abort — HA's
discovery dedup was dead, Python warned "coroutine ... was never awaited", and
the host-equality loop that used to sit here was the only accidental dedup.

The call is awaited and the now-unreachable host loop is gone (the real helper
is stricter: any entry for the domain, or any in-flight flow). The harness
mirrors the real contract instead of a sync no-op:

* `entity_stublib`'s hub stub makes the helper a coroutine (a sync stub would
  turn the production `await` into a TypeError);
* `test_standalone_setup`'s stub models the real semantics — a coroutine raising
  `DiscoveryDedupAbortError` (core raises AbortFlow), driven by the shared
  holder's entry list and in-progress list — and its tests now assert all three
  outcomes: awaited-and-proceeds (counter + claimed discovery unique id),
  `already_configured` when entries exist, `already_in_progress` when another
  flow runs. A fourth test collects the un-awaited-coroutine warning, and the
  prefill test in the UX suite asserts the awaited counter.

Red proof: with the pre-fix config_flow.py the repo-owned sweep reports
`config_flow.py:425: missing-await: self._async_handle_discovery_without_unique_id()`.

Verified: 289 passed, 2 skipped (D3 stage); ruff clean.

tests/entity_stublib.py

@@ -207,9 +207,13 @@ def install() -> None:
         async_set_unique_id = _stub_set_unique_id
         _abort_if_unique_id_configured = _noop_abort
 
-        def _async_handle_discovery_without_unique_id(self):
-            # t32 F2 (audited helper): recording no-op at hub scope
-            pass
+        async def _async_handle_discovery_without_unique_id(self):
+            # t37 D3: the real helper is a COROUTINE and core awaits it — a
+            # sync stub here would turn the production `await` into a
+            # TypeError. The abort semantics live in
+            # tests/test_standalone_setup.py, whose stub owns entry/in-progress
+            # state; at hub scope there is nothing to abort on.
+            return None
 
         def async_show_form(
             self, *, step_id, data_schema=None, errors=None,