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.sql.go

@@ -3814,6 +3814,72 @@ func (q *sqlQuerier) GetChatMessagesByChatID(ctx context.Context, arg GetChatMes
 	return items, nil
 }
 
+const getChatMessagesByChatIDDescPaginated = `-- name: GetChatMessagesByChatIDDescPaginated :many
+SELECT
+    id, chat_id, model_config_id, created_at, role, content, visibility, input_tokens, output_tokens, total_tokens, reasoning_tokens, cache_creation_tokens, cache_read_tokens, context_limit, compressed, created_by, content_version, total_cost_micros
+FROM
+    chat_messages
+WHERE
+    chat_id = $1::uuid
+    AND CASE
+        WHEN $2::bigint > 0 THEN id < $2::bigint
+        ELSE true
+    END
+    AND visibility IN ('user', 'both')
+ORDER BY
+    id DESC
+LIMIT
+    COALESCE(NULLIF($3::int, 0), 50)
+`
+
+type GetChatMessagesByChatIDDescPaginatedParams struct {
+	ChatID   uuid.UUID `db:"chat_id" json:"chat_id"`
+	BeforeID int64     `db:"before_id" json:"before_id"`
+	LimitVal int32     `db:"limit_val" json:"limit_val"`
+}
+
+func (q *sqlQuerier) GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg GetChatMessagesByChatIDDescPaginatedParams) ([]ChatMessage, error) {
+	rows, err := q.db.QueryContext(ctx, getChatMessagesByChatIDDescPaginated, arg.ChatID, arg.BeforeID, arg.LimitVal)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ChatMessage
+	for rows.Next() {
+		var i ChatMessage
+		if err := rows.Scan(
+			&i.ID,
+			&i.ChatID,
+			&i.ModelConfigID,
+			&i.CreatedAt,
+			&i.Role,
+			&i.Content,
+			&i.Visibility,
+			&i.InputTokens,
+			&i.OutputTokens,
+			&i.TotalTokens,
+			&i.ReasoningTokens,
+			&i.CacheCreationTokens,
+			&i.CacheReadTokens,
+			&i.ContextLimit,
+			&i.Compressed,
+			&i.CreatedBy,
+			&i.ContentVersion,
+			&i.TotalCostMicros,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const getChatMessagesForPromptByChatID = `-- name: GetChatMessagesForPromptByChatID :many
 WITH latest_compressed_summary AS (
     SELECT