bug(graph): literal If over an axis reopened after a Collect drops items #9917

Summary

A literal If whose selected branch iterates an axis that was reopened after a Collect gives wrong groups when the reopened collection has a different length from the original axis. An item is dropped silently and one group comes out empty. No error is raised.

Reproduction

Nodes (the test nodes are from tests/test_nodes.py):

• srcA: CollectionConcatInvocation(first=["a1", "a2"]) → itA: IterateInvocation
• itA.item → b: Mark("b:") → cc: CollectInvocation
• cc.collection → ext: CollectionConcatInvocation(second=["x"]) → itB: IterateInvocation (3 items)
• itB.item → t: Mark("t:") → if.true_input
• itA.item → f: Mark("f:") → if.false_input
• if: IfInvocation(condition=True) → collect: CollectInvocation

Run it with GraphExecutionState on main (seen at ac237a1, on both schedulers).

Observed: the graph completes with three collect outputs, at paths [0] → ["t:b:a1"], [1] → ["t:b:a2"] and [2] → []. Item x never reaches t, so t:x is missing.

Expected: t:x is not lost. Either the selected branch's items are collected correctly, or the shape is rejected with UnsupportedIfIterationAxesError before anything runs, as other unsupported axis shapes are.

With condition=False the same graph is rejected with UnsupportedIfIterationAxesError once #9789 is merged.

Notes

One narrow rule would turn this into an error: allow a reopened axis only when the new Iterate reads straight from the closing Collect. That changes the result of a graph main currently completes, so it was left out of #9789 and is waiting on a maintainer decision there.

Found during review of #9789.