Add behavioral sync test suite and fix ETag-fallback conflict bug

[D-VR]

Adds a behavioral test suite for the sync pipeline and fixes a conflict-detection bug surfaced while writing it.

Tests

• HTTP fixture harness (FakeSession/FakeResponse) with route registration, call counting, and a guard that fails on unexpected requests.
• Fixture- and snapshot-driven coverage of the main modules: nested folders, Opencast LTI single/series, Sciebo PROPFIND caching, skip rules, and a mixed-module course; plus course/semester filtering and storage safety.
• download_file behavior tests: chunked-to-disk download, in-run path dedup, and every update_files_conflict mode (keep/none/overwrite/rename + unknown->rename).
• assert_snapshot helper with SMM_UPDATE_SNAPSHOTS=1 to regenerate snapshots.
• Converted test_course_prefix_handling from unittest to pytest style for consistency.

Fix

• download_file: a failed local-vs-cache ETag comparison no longer silently overwrites the local file. A faulty/unreadable ETag cache is now treated as if no cached ETag existed, falling back to the timestamp/HEAD heuristic to decide whether the local copy was modified.

Closes #73