fix(auto_debug): align test expectation with fail-safe LLM exception handling

The _validate_fix BDD scenario "Validate fix handles LLM invocation
failure gracefully" was asserting fix_validated=True (fail-open), but
the code already sets is_valid=False on LLM exception (fail-safe).
Update the scenario step to "the fix should not be marked as validated"
and remove the pragma: no cover comment since this branch is now
exercised by the test.

ISSUES CLOSED: #10496

features/auto_debug_coverage_boost.feature

@@ -100,7 +100,7 @@ Feature: AutoDebug graph node coverage boost
     Given an auto debug agent with a failing LLM
     And a state with a current fix to validate
     When I call _validate_fix with the prepared state
-    Then the fix should be marked as validated
+    Then the fix should not be marked as validated
 
   # -----------------------------------------------------------------------
   # _should_retry_fix routing