Product Search and Filter

You are building search helpers for a small product list. Some helpers should return multiple products, and one helper should return a single product.

1. `filterByCategory(products, category)` should return products in the matching category.
2. `filterByMaxPrice(products, maxPrice)` should return products at or below the max price.
3. `getInStockProducts(products)` should return products where `inStock` is `true`.
4. `findProductById(products, productId)` should return one matching product or `undefined`.
5. `searchProducts(products, searchText)` should return products whose name includes the search text, ignoring casing.