fix: LazyScanStream prepare/execute is CPU-work not I/O (#9078) #6585

Job	Run time
Rust/C++ FFI tests (asan)	2m 10s
Rust tests (tsan)	6m 41s
DuckDB libraries available in R2	4s
Rust tests (asan)	7m 0s
Rust tests (miri)	1m 34s
C++ API (coverage)	1m 4s
Rust/C++ FFI tests (tsan)	1m 59s
Mirror DuckDB to R2	0s
Rust tests (coverage) (${{ matrix.suite }})	0s
	20m 32s

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