fix(autofmt): don't duplicate pretty continuation lines after multi-line source match

[Not-Sarthak]

Summary

Fixes #5523.

dx fmt corrupted RSX event handler blocks containing #[cfg]-gated let bindings around a move |_| closure. When prettier_please broke a long statement at a different line position than the source did, the multi-line accumulator in write_partial_expr consumed every source line for the logical statement but the outer iterator kept walking pretty lines, re-emitted the trailing continuation (.clone();), and advanced the source pointer past unrelated statements — producing invalid Rust:

let X = Y.clone();
    .clone();
if let Err(err) = ... {
    err
    ));

Fix

pretty.lines() → peekable. After emitting a multi-line verbatim source block whose accumulated content extends beyond the matched pretty line, consume the pretty continuation lines whose content is already covered.

Test plan

• New fixture tests/samples/cfg_let_event_block.rsx wired into twoway!
• New cfg_let_event_block_is_idempotent test
• 84 existing autofmt tests pass
• cargo clippy --workspace --all-features --all-targets -- -D warnings clean
• cargo fmt --all -- --check clean
• cargo test --workspace --exclude dioxus-desktop clean (334 test bins, 0 failures)
• Stress-tested 8 adversarial inputs (cfg-lets in various positions, async closures, long method chains, format!() macros) — all produce valid Rust