fix(autofmt): stop dx fmt --all-code from duplicating and truncating …

[chairulakmal]

…closure bodies

Fixes #5682.

dx fmt --all-code runs the file through prettyplease before formatting the rsx! bodies, and prettyplease often wraps a closure in the middle of its signature. write_partial_expr then reads the wrapped line as a hand-written method chain, copies it out verbatim, and re-emits the lines after it, so the closure body comes out duplicated and truncated and the file no longer compiles.

This narrows that part of the code to only take over when the source it copied matches the formatter output it replaces exactly.

Swept over every .rs file in examples/ and packages/: plain dx fmt output does not change, and the 24 files that --all-code corrupts today (20 of them into text syn cannot parse) all format cleanly with this change.