Fix the Broken Search Filter in a Book Catalog API

Problem Statement

You are building a backend search service for a bookstore catalog. You are given a list of books, where each book is a dictionary with title, author, and genre fields. Your task is to implement a filter_books function that returns a list of books matching a search query.

Requirements:

1. The search query should be case-insensitive.
2. The search query should be checked against the title, author, and genre fields.
3. The search query should be matched as a substring (i.e., if the query is "art", a book with genre "Fantasy" would be a match if it contained the sequence "art").
4. You must trim leading and trailing whitespace from the search query before performing the search.
5. If the search query is empty (after trimming), return all books in the catalog.

Function Signature:

def filter_books(catalog, query):
    # catalog: list of dicts [{'title': str, 'author': str, 'genre': str}]
    # query: str
    # returns: list of dicts

Example:

catalog = [
    {'title': 'The Great Gatsby', 'author': 'F. Scott Fitzgerald', 'genre': 'Classic'},
    {'title': 'Art of War', 'author': 'Sun Tzu', 'genre': 'Philosophy'}
]

# Query "art"
# Output: [
#    {'title': 'The Great Gatsby', 'author': 'F. Scott Fitzgerald', 'genre': 'Classic'},
#    {'title': 'Art of War', 'author': 'Sun Tzu', 'genre': 'Philosophy'}
# ]
# Explanation: "Art" matches the end of "Gatsby" (case-insensitive) and the beginning of "Art of War".

Constraints:

• catalog length is between 0 and 10,000.
• query length is between 0 and 100.
• Fields are always non-null strings.