Respect `msgspec.Struct` default-copy semantics #19658

Job	Run time
cargo clippy	2m 27s
benchmarks	13m 40s
cargo fmt	19s
cargo fuzz	7m 31s
cargo test \| ubuntu-latest	7m 32s
cargo test (wasm)	1m 28s
cargo udeps	2m 54s
Determine changes	13s
cargo test \| windows-latest	12m 4s
mkdocs	2m 28s
pre-commit	2m 15s
python package	1m 42s
test scripts	1m 0s
Formatter ecosystem and progress checks	0s
ecosystem	5m 58s
	1h 1m 31s

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