Reject cyclic nested end-to-end flows

[lwrage]

Summary

• track active end-to-end flow declarations across recursive nested-flow instantiation
• detect back-references, report a cycle, and propagate failure to dependent flows
• remove partial instances so cyclic or incomplete instance graphs are not retained
• guard the regression with a 30-second timeout against unbounded recursion

Testing

• before the fix, Issue2987Test timed out after 30 seconds
• after the fix, the timeout-guarded regression completed in about 3 seconds
• focused end-to-end-flow regressions: 3 tests passed
• complete org.osate.core.tests bundle: 541 tests passed

Fixes #2987