PHOENIX-7830 De-flake MutableIndexFailureIT

[apurtell]

MutableIndexFailureIT has two independent bugs that cause flaky and cascading test failures. First, the outer class declares its own static volatile FAIL_WRITE field that shadows FailingRegionObserver.FAIL_WRITE, so the finally block in testIndexWriteFailure resets the dead outer field and leaves FailingRegionObserver.FAIL_WRITE stuck at true when the test fails mid-way, breaking subsequent tests in the same fork. Second, FailingRegionObserver.TOGGLE_FAIL_WRITE_FOR_RETRY is not declared volatile even though it is written by app threads in addRowsInTableDuringRetry and read by region server handler threads in preBatchMutate, so the RS thread can observe a stale false, miss the retry toggle, and surface a CommitException that fails the assertion. The fix removes the shadowed outer FAIL_WRITE field, retargets the finally block to reset FailingRegionObserver.FAIL_WRITE, and marks FailingRegionObserver.TOGGLE_FAIL_WRITE_FOR_RETRY volatile to guarantee cross-thread visibility.