Fix graph stall for collectors downstream of empty iterators

[JPPhoto]

Summary

Fixes graph execution stalling when a Collect node is downstream of an iterator over an empty collection.

Zero-iteration expansions are now explicitly recorded as complete. This allows downstream collectors to execute with an empty collection, typed consumers to receive [], and the session to report completion.

Collector output validation now handles materialized collectors with no runtime input edges. In this case, the already-validated source graph remains authoritative for type compatibility.

Related Issues / Discussions

Closes #9347

QA Instructions

Added coverage for:

• IntegerCollection([]) -> Iterate -> Add -> Collect -> typed consumer
• Nested empty Iterate -> Collect -> Iterate -> Collect chains
• Collector output and typed consumer both receiving []
• GraphExecutionState.is_complete() returning True

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)