Fix just_from submit exception specification

[fallintoplace]

What changed

• Make both just_from::submit() overloads conditionally noexcept.
• Add coverage for a callable that throws while being moved during submit().

Why

submit() calls connect(), but both overloads were unconditionally noexcept. A throwing callable move could therefore terminate instead of propagating the exception.

Testing

• build/test/exec/test.exec [just_from]
• ctest --test-dir build --output-on-failure -j 8

All 953 configured tests passed.

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[ericniebler]

receivers are now required to be nothrow move constructible, so this check is unnecessary.

Suggested change

	auto submit(Rcvr rcvr) && noexcept(STDEXEC::__nothrow_decay_copyable<Rcvr, Fn>) -> void
	auto submit(Rcvr rcvr) && noexcept(STDEXEC::__nothrow_decay_copyable<Fn>) -> void

[ericniebler]

Suggested change

	auto submit(Rcvr rcvr) const & noexcept(STDEXEC::__nothrow_decay_copyable<Rcvr, Fn const &>)
	auto submit(Rcvr rcvr) const & noexcept(STDEXEC::__nothrow_decay_copyable<Fn const &>)

[ericniebler]

/ok to test 5742a1f