Add test case for coercion after traversal inside `@optional` scope. #1725

Job	Run time
Check rustdoc WASM target	29s
Run JS linters	45s
Check lint and rustfmt	33s
Check fuzz targets	10s
Run tests	42s
WASM tests	1m 21s
Python tests and maturin build (3.9)	38s
Python tests and maturin build (3.10)	50s
pre-publish checks	0s
Python tests and maturin build (3.11)	59s
Python tests and maturin build (3.12)	57s
Python tests and maturin build (3.13.0-rc - 3.13)	58s
All CI stages	1s
pre-publish checks	0s
publish trustfall_filetests_macros	0s
publish trustfall_core	0s
publish trustfall_derive	0s
publish python trustfall	0s
publish trustfall	0s
publish trustfall_stubgen	0s
	8m 23s

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