Fix schedule_all completion for empty ranges

[fallintoplace]

Summary

Fix exec::schedule_all on static_thread_pool so empty ranges complete immediately.

countdown_ is initialized from the range size. For an empty range, no child operation is started, so nothing decrements the countdown or completes the final receiver. The operation now sends set_value directly when there are no items to schedule.

Testing

• cmake --build build --target test.exec
• ./build/test/exec/test.exec "schedule_all on static_thread_pool handles empty ranges"
• ./build/test/exec/test.exec "[types][static_thread_pool]"
• ASAN_OPTIONS=abort_on_error=0:symbolize=1 ./build-asan/test/exec/test.exec "schedule_all on static_thread_pool handles empty ranges"
• ./build/test/exec/test.exec

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