docs(tests): refresh test_backoff.py docstring post-merge

Stale references to "Task 7" and "through Tasks 6-7" don't survive
the shipped branch. Reword to describe what the file actually does
(unit coverage of the pure helper; integration via Retry tests).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512

tests/test_backoff.py

@@ -1,7 +1,8 @@
-"""Unit test for the full-jitter backoff helper.
+"""Unit tests for the full-jitter backoff helper.
 
-Integration via Retry middleware lands in Task 7's tests; this file exists
-only to keep the 100% coverage gate passing through Tasks 6-7.
+Integration coverage comes from ``tests/test_retry.py`` (Retry middleware drives
+``full_jitter_delay`` per attempt). The pure-function tests here pin the bound
+and the cap independently of the middleware orchestration.
 """
 
 from httpware.middleware.resilience._backoff import full_jitter_delay