Refactor integration tests to use feedparser mocking

Replace responses library HTTP mocking with direct feedparser.parse()
mocking to eliminate XML parsing compatibility issues. All integration
tests now mock feedparser output directly, avoiding HTTP layer complexity.

Changes:
- Replace responses.activate decorators with feedparser.parse patches
- Mock feed objects directly instead of mocking HTTP responses
- Remove responses library dependency from requirements-test.txt
- Simplify test setup by eliminating XML string encoding issues

This approach provides more reliable testing by directly controlling
feedparser behavior rather than relying on HTTP mocking layer.

requirements-test.txt

@@ -9,9 +9,6 @@ pytest==7.4.4
 pytest-cov==4.1.0
 pytest-mock==3.12.0
 
-# HTTP mocking for integration tests
-responses==0.24.1
-
 # Code quality
 flake8==7.0.0
 black==24.1.1