Fix read_with_default connect noexcept

[fallintoplace]

What changed

• Make read_with_default member connect() derive its exception specification from constructing the returned operation state.
• Add a regression for a default value whose move throws during connect().

Why

The operation state already accounts for a potentially throwing default move, but connect() only considered moving the receiver.

Testing

• build/test/exec/test.exec (3345 assertions passed)
• ctest --test-dir build --output-on-failure -j 8 (948 passed)

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