fix(django-cf): return a fresh new cursor for every cursor call

[ryanking13]

Fixes a bug that calling cursor() against a database connection was returning always the same cursor.

cursorA = connection.cursor()
cursorB = connection.cursor()

assert cursorA != cursorB  # should not be the same!

[ask-bonk]

The change looks clean and correct. The PR:

1. Extracts cursor behavior into a separate CFCursor class so CFDatabase.cursor() returns a fresh instance each call, fixing the shared-cursor bug.
2. CFCursor correctly proxies _defer_foreign_keys to the shared CFDatabase state.
3. Tests are thorough — they cover cursor independence, shared defer_foreign_keys state, and a real-world nested queryset scenario.
4. The test_iterator_chunk_size_one_survives_nested_foreign_key_queries test is an excellent integration test that validates the exact bug scenario.

LGTM

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