Replace four time.sleep(0.01) timestamp guards in
memory_service_coverage_steps.py with a deterministic
_wait_for_clock_advance() helper that busy-waits on the monotonic
clock until the UTC clock actually advances past the recorded
before timestamp, bounded by a 2-second deadline.

Also introduce memory_service_clock_wait.feature and its step
definitions to verify the helper raises AssertionError on deadline
exceeded and returns normally once the clock advances.

Closes #9963

@@ -0,0 +13,4 @@
+  # deadline is exceeded.
+
+  @mock_only
+  Scenario: Clock-advance guard raises AssertionError when deadline is exceeded

@@ -0,0 +19,4 @@
+    Then an AssertionError should be raised mentioning the deadline
+
+  @mock_only
+  Scenario: Clock-advance guard returns successfully once the clock advances