loadscope: forget a dead worker's collection so a still-collecting worker isn't mistaken for done

[dchaudhari7177]

Fixes #1362.

Problem

LoadScopeScheduling.remove_node() pops the dead worker from assigned_work but never removes its entry from registered_collections. Since collection_is_completed is len(registered_collections) >= numnodes, the stale entry keeps the collection looking "complete" while a later worker is still collecting. schedule() then runs and _assign_work_unit() raises KeyError indexing registered_collections for the worker that never registered — crashing the run with INTERNALERROR. This affects both --dist=loadscope and --dist=loadfile (the latter subclasses LoadScopeScheduling).

Fix

Drop the node from registered_collections in remove_node(), mirroring the assigned_work.pop(node) already there. A replacement worker then re-registers normally, and collection_is_completed correctly waits for every live worker.

This is safe for the shutdown path too: normal worker_workerfinished removals only happen after triggershutdown, and on a crash the replacement worker re-registers its collection.

Test

Added TestLoadScopeScheduling::test_remove_node_forgets_dead_worker_collection, which reproduces the scenario with MockNodes (3 expected workers; one dies mid-collection while another is still collecting). It fails on main (registered_collections still holds the dead node, collection_is_completed flips true early) and passes with the fix. Full testing/test_dsession.py is green; ruff check/format clean. Added changelog/1362.bugfix.rst.