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/database/dump.sql

@@ -362,10 +362,7 @@ CREATE TYPE chat_reasoning_effort AS ENUM (
 
 CREATE TYPE chat_status AS ENUM (
     'waiting',
-    'pending',
     'running',
-    'paused',
-    'completed',
     'error',
     'requires_action',
     'interrupting'
@@ -4763,8 +4760,6 @@ CREATE INDEX idx_chats_owner ON chats USING btree (owner_id);
 
 CREATE INDEX idx_chats_parent_chat_id ON chats USING btree (parent_chat_id);
 
-CREATE INDEX idx_chats_pending ON chats USING btree (status) WHERE (status = 'pending'::chat_status);
-
 CREATE INDEX idx_chats_root_chat_id ON chats USING btree (root_chat_id);
 
 CREATE INDEX idx_chats_worker_acquisition_candidates ON chats USING btree (status, updated_at, id) WHERE (archived = false);