feat: remove unused chat statuses pending, paused, and completed (#27064)

The chatd state machine only recognizes `waiting`, `running`, `error`,
`requires_action`, and `interrupting`. Remove the unused `pending`,
`paused`, and `completed` values from the database enum, backend, SDK,
frontend, generated queries, and API docs.

Migration `000543_chat_status_remove_unused` remaps existing `pending`
rows to `running`, remaps `paused` and `completed` rows to `waiting`,
drops the obsolete `idx_chats_pending` index, and recreates
`chats_expanded` around the enum swap. It also removes the dead
`AcquireChats` query and all remaining query literals for the deleted
statuses.

**NOTE**: The enum swap can break chat queries from older replicas
during a mixed-version rollout because they still reference
`'pending'::chat_status`. Chats are experimental, so this PR accepts
that limited rollout window instead of adding a two-release expand and
contract sequence.

> This PR was authored by Mux (AI agent) on Mike's behalf.

coderd/coderdtest/chat.go

@@ -117,7 +117,7 @@ func waitForChatTerminalState(
 		if err != nil {
 			return false
 		}
-		return chat.Status != database.ChatStatusPending && chat.Status != database.ChatStatusRunning
+		return chat.Status != database.ChatStatusRunning
 	}, testutil.WaitLong, testutil.IntervalFast)
 }