[bug]: If inside a For body cannot select root-frame branch values (hang on main, KeyError with #9789) #9818

Is there an existing issue for this problem?

• I have searched the existing issues

Version number

main at aa46003 (7.0.0-alpha.2-post), and #9789 at e27f6fd

What happened

Consider an If inside a For body whose condition is the loop item, with root-frame nodes (nodes with no loop input) on its branches. On main the session hangs right after for runs: next() returns None and is_complete() stays False. With #9789 it fails with KeyError: 'root_true' or KeyError: 'root_false' from create_for_body_iteration.

This happens for every condition sequence, including [False, False] and [True, True].

What you expected to happen

Each iteration should return the selected root value, for example ["false-root", "true-root"] for [False, True]. A root branch value should run once, and only if some iteration selects it.

How to reproduce the problem

g = Graph()
g.add_node(ForInvocation(id="for", collection=[False, True]))
g.add_node(AnyTypeTestInvocation(id="root_true", value="true-root"))
g.add_node(AnyTypeTestInvocation(id="root_false", value="false-root"))
g.add_node(IfInvocation(id="if"))
g.add_node(ForReturnInvocation(id="return"))
g.add_node(AnyTypeTestInvocation(id="after"))
g.add_edge(create_edge("for", "item", "if", "condition"))
g.add_edge(create_edge("root_true", "value", "if", "true_input"))
g.add_edge(create_edge("root_false", "value", "if", "false_input"))
g.add_edge(create_edge("if", "value", "return", "output"))
g.add_edge(create_loop_linkage("for", "return"))
g.add_edge(create_edge("for", "output_collection", "after", "value"))

Run it with the _run_graph helper from tests/app/services/shared/test_execution_engine_differential.py. The helpers come from tests/test_nodes.py.

Additional context

The For body planner creates each iteration without an admission check and expects every body input to be prepared already. A root-frame branch source is prepared only when some If execution selects it, and those executions appear one iteration at a time.

The same shape with an Iterate instead of a For works with #9789. Any fix also needs to make sure that once every iteration has resolved, an unselected root source is rejected rather than run.

Found while reviewing #9789 (follow-up to #9484).