fix(graph): validate convergence targets in addParallelConditionalEdges

[YuqiGuo105]

Summary

Fixes #4411

When using addParallelConditionalEdges(), if different route branches' mapped nodes have edges pointing to different convergence targets, findParallelNodeTargets() returns a Set with multiple elements. HashSet.iterator().next() then silently picks one non-deterministically, causing the ConditionalParallelNode to route to an unpredictable next node after parallel execution.

Root Cause

In CompiledGraph constructor, when processing conditional parallel edges:

1. findParallelNodeTargets() collects each route branch node's edge target into a Set<String>
2. If node_a -> convergence and node_b -> some_other_node, the Set contains 2 elements
3. parallelNodeTargets.iterator().next() picks whichever element HashSet happens to return first - this is non-deterministic

Fix

Added compile-time validation in CompiledGraph: when parallelNodeTargets.size() > 1, throw GraphStateException with a clear error message listing the conflicting convergence targets, rather than silently picking a random one.

This approach aligns with the issue reporter's suggestion: 'findParallelNodeTargets returns multiple targets should throw GraphStateException'

Changes

File	Change
CompiledGraph.java	Added size() > 1 validation check before iterator().next()
Errors.java	Added inconsistentConvergenceTargetsOnConditionalParallelEdge error constant
StateGraphTest.java	Added regression test with inconsistent convergence targets

Test

All 4 testAddParallelConditionalEdges* tests pass (BUILD SUCCESS).

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