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/queries/chats.sql

@@ -1469,7 +1469,7 @@ UPDATE chats
 SET context_dirty_since = @dirty_since
 WHERE agent_id = @agent_id::uuid
     AND archived = false
-    AND status IN ('waiting', 'running', 'paused', 'pending', 'requires_action')
+    AND status IN ('waiting', 'running', 'requires_action')
     AND context_aggregate_hash IS NOT NULL
     AND context_aggregate_hash IS DISTINCT FROM @aggregate_hash
     AND context_dirty_since IS NULL
@@ -1540,90 +1540,6 @@ SELECT
     (SELECT COUNT(*)::int FROM genuinely_new) -
     (SELECT COUNT(*)::int FROM inserted) AS rejected_new_files;
 
--- name: AcquireChats :many
--- Acquires up to @num_chats pending chats for processing. Uses SKIP LOCKED
--- to prevent multiple replicas from acquiring the same chat.
-WITH acquired_chats AS (
-UPDATE
-    chats
-SET
-    status = 'running'::chat_status,
-    started_at = @started_at::timestamptz,
-    heartbeat_at = @started_at::timestamptz,
-    updated_at = @started_at::timestamptz,
-    worker_id = @worker_id::uuid
-WHERE
-    id = ANY(
-        SELECT
-            id
-        FROM
-            chats
-        WHERE
-            status = 'pending'::chat_status
-            AND archived = false
-        ORDER BY
-            updated_at ASC
-        FOR UPDATE
-            SKIP LOCKED
-        LIMIT
-            @num_chats::int
-    )
-RETURNING *
-),
-chats_expanded AS (
-    SELECT
-        acquired_chats.id,
-        acquired_chats.owner_id,
-        acquired_chats.workspace_id,
-        acquired_chats.title,
-        acquired_chats.status,
-        acquired_chats.worker_id,
-        acquired_chats.started_at,
-        acquired_chats.heartbeat_at,
-        acquired_chats.created_at,
-        acquired_chats.updated_at,
-        acquired_chats.parent_chat_id,
-        acquired_chats.root_chat_id,
-        acquired_chats.last_model_config_id,
-        acquired_chats.last_reasoning_effort,
-        acquired_chats.archived,
-        acquired_chats.last_error,
-        acquired_chats.mode,
-        acquired_chats.mcp_server_ids,
-        acquired_chats.labels,
-        acquired_chats.build_id,
-        acquired_chats.agent_id,
-        acquired_chats.pin_order,
-        acquired_chats.last_read_message_id,
-        acquired_chats.dynamic_tools,
-        acquired_chats.organization_id,
-        acquired_chats.plan_mode,
-        acquired_chats.client_type,
-        acquired_chats.last_turn_summary,
-        acquired_chats.snapshot_version,
-        acquired_chats.history_version,
-        acquired_chats.queue_version,
-        acquired_chats.generation_attempt,
-        acquired_chats.retry_state,
-        acquired_chats.retry_state_version,
-        acquired_chats.runner_id,
-        acquired_chats.requires_action_deadline_at,
-        COALESCE(root.user_acl, acquired_chats.user_acl) AS user_acl,
-        COALESCE(root.group_acl, acquired_chats.group_acl) AS group_acl,
-        owner.username AS owner_username,
-        owner.name AS owner_name,
-        acquired_chats.context_aggregate_hash,
-        acquired_chats.context_dirty_since,
-        acquired_chats.context_dirty_resources,
-        acquired_chats.context_error
-    FROM
-        acquired_chats
-    LEFT JOIN chats root ON root.id = COALESCE(acquired_chats.root_chat_id, acquired_chats.parent_chat_id)
-    JOIN visible_users owner ON owner.id = acquired_chats.owner_id
-)
-SELECT *
-FROM chats_expanded;
-
 -- name: UpdateChatStatus :one
 WITH updated_chat AS (
 UPDATE
@@ -2534,10 +2450,9 @@ SELECT *
 FROM chats_expanded
 WHERE agent_id = @agent_id::uuid
     AND archived = false
-    -- Active statuses only: waiting, pending, running, paused,
-    -- requires_action.
-    -- Excludes completed and error (terminal states).
-    AND status IN ('waiting', 'running', 'paused', 'pending', 'requires_action')
+    -- Active statuses only: waiting, running, requires_action.
+    -- Excludes error (terminal state) and interrupting.
+    AND status IN ('waiting', 'running', 'requires_action')
 ORDER BY updated_at DESC;
 
 -- name: SoftDeleteContextFileMessages :exec
@@ -2630,8 +2545,6 @@ WHERE
     AND chats_expanded.status NOT IN (
         'running'::chat_status,
         'interrupting'::chat_status,
-        'pending'::chat_status,
-        'paused'::chat_status,
         'requires_action'::chat_status
     )
     AND COALESCE(activity.last_activity_at, chats_expanded.created_at) < @archive_cutoff::timestamptz
@@ -2999,7 +2912,7 @@ WITH to_archive AS (
       -- Redundant filter helps the planner use the partial index on created_at.
       AND c.created_at < @archive_cutoff::timestamptz
       -- New active statuses must be added here to prevent archiving.
-      AND c.status NOT IN ('running', 'pending', 'paused', 'requires_action')
+      AND c.status NOT IN ('running', 'requires_action')
       AND COALESCE(activity.last_activity_at, c.created_at) < @archive_cutoff::timestamptz
     -- Sorting by created_at lets Postgres drive the scan from the
     -- partial index instead of evaluating every LATERAL subquery