Fix when_any error completion decay

[fallintoplace]

What changed

• Decay set_value and set_error arguments in when_any completion signatures.
• Advertise set_error(std::exception_ptr) when storing any completion can throw.
• Add regressions for lvalue error delivery and throwing error decay.

Why

when_any stores completion arguments in decayed tuples and forwards the winner from moved storage. The completion signatures now match that storage, including its std::exception_ptr fallback when decay-copying throws.

Testing

• build/test/exec/test.exec (3345 assertions, 338 test cases)
• ctest --test-dir build --output-on-failure -j 1 (949 passed)

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