`format_code_in_doc_comments` adds empty line between trailing comment and hidden code

[jdygert-spok]

/// ```
/// println!("1"); // comment
/// # println!("2")
/// ```

becomes

/// ```
/// println!("1"); // comment
///
/// # println!("2")
/// ```

rustfmt 1.8.0-nightly (9748d87dc7 2025-07-21)

Parent: #3348

[matthewhughes934]

this isn't limited to formatting code in comments: the empty line is added whenever the comment is followed by a comment of another style, input:

fn wat() {
    println!("1"); // commment
    /* # println!("2") */
}

Needs 2024 edition to reproduce:

$ rustfmt --verbose --check --edition 2024 sample.rs 
Formatting /tmp/sample.rs
Diff in /tmp/sample.rs:1:
 fn wat() {⏎
     println!("1"); // commment⏎
+    ⏎
     /* # println!("2") */⏎
 }⏎
 ⏎

the issue with the code block is that internally it's transformed to:

println!("1"); // comment
//### # println!("2")

i.e. comment with a different style

[matthewhughes934]

#6617 addresses this