Fixed test_execution_context_can_enter which had a logic error (expected
to re-enter object 1 while it was still in the stack). Added three new
tests for acceptance criteria:
- test_execution_context_nested_cycle_a_b_a: A->B->A cycle detection
- test_execution_context_sequential_invocation: same form twice sequentially
- test_execution_context_diamond_pattern: A->B and A->C->D, B and C both invoke D
All 7 execution_context tests pass. The cycle detection infrastructure
(ExecutionContext, can_enter/enter/exit, diagnostic codes) was already
implemented; this commit fixes the test bug and adds missing coverage.
Closes: pdftract-27tu5
