test(sequencing): Add SB.2.9 specification compliance tests

Add tests for constrainChoice and preventActivation validation:
- Backward choice to incomplete activity with constrainChoice
- Choice to unattempted activity with preventActivation
- Multi-level ancestor constraint validation
- Positive cases for allowed navigation scenarios

Fix test setup issue where isActive was set before currentActivity
setter (which auto-activates), causing tests to hit wrong code path.

Note: Tests hit lines 569-578 in choiceSequencingRequestProcess.
Lines 2932-2937 in validateConstraintsAtAncestorLevel remain
uncovered due to duplicated validation logic in the codebase.

Author: jcputney