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/httpmw/chatparam_test.go

@@ -62,6 +62,7 @@ func TestChatParam(t *testing.T) {
 		require.NoError(t, err)
 
 		chat, err := db.InsertChat(context.Background(), database.InsertChatParams{
+			Status:            database.ChatStatusWaiting,
 			OwnerID:           ownerID,
 			WorkspaceID:       uuid.NullUUID{},
 			ParentChatID:      uuid.NullUUID{},