refactor(coderd/x/chatd): insert chats directly as pending on creation (#23888)

Previously, `CreateChat` inserted the `chats` row with the DB default
status (`waiting`), then updated it to `pending` in the same transaction
via `setChatPendingWithStore`. This wasted two extra queries per chat
creation (`GetChatByID` + `UpdateChatStatus`) and rewrote the same row
immediately after inserting it.

Now `CreateChat` passes the status directly to `InsertChat`, so the row
is written once in its final create-time state. The
`setChatPendingWithStore` helper is removed entirely. `InsertChat` now
requires an explicit `status` parameter at all callsites instead of
relying on a DB column default.

## Motivation

On an experimental branch we're trialing firing all chatd notifications
from plpgsql triggers. The old two-step insert made that awkward: in an
`AFTER INSERT` trigger, `NEW` only contained the insert-time row
(`waiting`), not the final committed state (`pending`). To emit the
correct event payload the trigger had to be deferred and re-read the row
from `chats` at commit time.

With this change, `NEW` already contains the correct row to publish — no
deferred trigger, no extra `SELECT`, simpler and cheaper trigger logic.

That said, this seems like a worthwhile change regardless of the trigger
experiment: writing the final row state once removes unnecessary DB work
on every chat creation and makes the create path easier to reason about.

coderd/database/dbauthz/dbauthz_test.go

@@ -721,7 +721,9 @@ func (s *MethodTestSuite) TestChats() {
 		check.Args(threshold).Asserts(rbac.ResourceChat, policy.ActionRead).Returns(chats)
 	}))
 	s.Run("InsertChat", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
-		arg := testutil.Fake(s.T(), faker, database.InsertChatParams{})
+		arg := testutil.Fake(s.T(), faker, database.InsertChatParams{
+			Status: database.ChatStatusWaiting,
+		})
 		chat := testutil.Fake(s.T(), faker, database.Chat{OwnerID: arg.OwnerID})
 		dbm.EXPECT().InsertChat(gomock.Any(), arg).Return(chat, nil).AnyTimes()
 		check.Args(arg).Asserts(rbac.ResourceChat.WithOwner(arg.OwnerID.String()), policy.ActionCreate).Returns(chat)