style: rustfmt the custom_wiki2html test assert (fixes red CI)

The custom_wiki2html test added in 23aec3e had an over-width `assert!`
that `cargo fmt` wants wrapped; CI's `cargo fmt --all -- --check` job has
been failing on it since. No behavior change — `cargo fmt --all` only.

Verified locally against the exact CI matrix: fmt --check clean, clippy
--workspace --all-targets -D warnings clean, test --workspace --all-targets
all pass, keymap + config harnesses pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/tests/html_export.rs

@@ -561,7 +561,10 @@ fn write_page_invokes_custom_wiki2html_converter() {
         nuwiki_lsp::commands::export_ops::write_page(&parse("= Hello =\n"), "Hello", &c, true)
             .unwrap();
     assert_eq!(outcome.page, "Hello");
-    assert!(outcome.output_path.exists(), "converter must produce output");
+    assert!(
+        outcome.output_path.exists(),
+        "converter must produce output"
+    );
     let body = std::fs::read_to_string(&outcome.output_path).unwrap();
     assert!(
         body.starts_with("CUSTOM:"),