fix: call pg_notification_queue_usage() automatically when the LISTEN channel bug is detected (#4858)

There's a PostgreSQL bug that doesn't let any listener to register in the DB:
https://www.postgresql.org/message-id/flat/CAK98qZ3wZLE-RZJN_Y%2BTFjiTRPPFPBwNBpBi5K5CU8hUHkzDpw%40mail.gmail.com
The only workaround is to advance the async notification queue tail,
which can be done by executing:  "SELECT pg_notification_queue_usage();".
Before we just logged a HINT with this suggestion, but now we call that function directly
and then let the listener to automatically recover.

No automated tests were added here as it would be too complex and this is a PostgreSQL bug. But this was manually tested following the steps on https://github.com/PostgREST/postgrest/pull/4581#issuecomment-3690610592

src/PostgREST/Observation.hs

@@ -13,7 +13,6 @@ module PostgREST.Observation
   , observationMessage
   , ObservationHandler
   , showOnSingleLine
-  , isDbListenerBug
   ) where
 
 import qualified Data.ByteString.Lazy       as LBS
@@ -48,7 +47,7 @@ data Observation
   | DBListenStart (Maybe ByteString) (Maybe ByteString) Text Text -- host, port, version string, channel
   | DBListenFail Text (Either SQL.ConnectionError SomeException)
   | DBListenRetry Int
-  | DBListenBugHint -- https://github.com/PostgREST/postgrest/issues/3147
+  | DBListenBugCallQueryFix
   | DBListenerGotSCacheMsg ByteString
   | DBListenerGotConfigMsg ByteString
   | DBListenerConnectionCleanupFail SomeException
@@ -122,8 +121,8 @@ observationMessage = \case
       either showListenerConnError showListenerException listenErr
   DBListenRetry delay ->
     "Retrying listening for database notifications in " <> (show delay::Text) <> " seconds..."
-  DBListenBugHint ->
-    "HINT:  This is likely a bug in the notification queue, try executing the following to solve it: select pg_notification_queue_usage();"
+  DBListenBugCallQueryFix ->
+    "This is likely a PostgreSQL bug in the notification queue, executing the following to try to solve it: SELECT pg_notification_queue_usage();"
   DBListenerGotSCacheMsg channel ->
     "Received a schema cache reload message on the " <> show channel <> " channel"
   DBListenerGotConfigMsg channel ->
@@ -188,6 +187,3 @@ observationMessage = \case
 
 showOnSingleLine :: Char -> Text -> Text
 showOnSingleLine split txt = T.intercalate " " $ T.filter (/= split) <$> T.lines txt -- the errors from hasql-notifications come intercalated with "\t\n"
-
-isDbListenerBug :: SomeException -> Bool
-isDbListenerBug e = "could not access status of transaction" `T.isInfixOf` show e