feat: paginate chat messages endpoint with cursor-based infinite scroll (#23083)

Adds cursor-based pagination to the chat messages endpoint.

## Backend

- New `GetChatMessagesByChatIDPaginated` SQL query: returns messages in
`id DESC` order with a `before_id` keyset cursor and configurable
`limit`
- Handler parses `?before_id=N&limit=N` query params, uses the `LIMIT
N+1` trick to set `has_more` without a separate COUNT query
- Queued messages only returned on the first page (no cursor) since
they're always the most recent
- SDK client updated with `ChatMessagesPaginationOptions`
- Fully backward compatible: omitting params returns the 50 newest
messages

## Frontend

- Switches `getChatMessages` from `useQuery` to `useInfiniteQuery` with
cursor chaining via `getNextPageParam`
- Pages flattened and sorted by `id` ascending for chronological display
- `MessagesPaginationSentinel` component uses `IntersectionObserver`
(200px rootMargin prefetch) inside the existing `flex-col-reverse`
scroll container
- `flex-col-reverse` handles scroll anchoring natively when older
messages are prepended — no manual `scrollTop` adjustment needed (same
pattern as coder/blink)

## Why cursor-based instead of offset/limit

Offset-based pagination breaks when new messages arrive while paginating
backward (offsets shift, causing duplicates or missed messages). The
`before_id` cursor is stable regardless of inserts — each page is
deterministic.

coderd/database/queries/chats.sql

@@ -40,6 +40,23 @@ WHERE
 ORDER BY
     created_at ASC;
 
+-- name: GetChatMessagesByChatIDDescPaginated :many
+SELECT
+    *
+FROM
+    chat_messages
+WHERE
+    chat_id = @chat_id::uuid
+    AND CASE
+        WHEN @before_id::bigint > 0 THEN id < @before_id::bigint
+        ELSE true
+    END
+    AND visibility IN ('user', 'both')
+ORDER BY
+    id DESC
+LIMIT
+    COALESCE(NULLIF(@limit_val::int, 0), 50);
+
 -- name: GetChatMessagesForPromptByChatID :many
 WITH latest_compressed_summary AS (
     SELECT