Fix nvexec upon_stopped move-only callable forwarding

[fallintoplace]

Stacked on #2183. The first commit comes from that PR, so the diff should shrink after it merges.

Summary

• preserve the sender value category when passing the upon_stopped handler into its receiver
• add a regression test with a trivially copyable, move-only handler
• keep lvalue senders copying their handlers

Testing

• added a focused nvexec regression test
• checked the new changes with clang-format 21
• ran git diff --check

[copy-pr-bot]

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