`normalize_comments = true` causes malformed comments

Input:

struct A {
    x: usize,
    /* protected int[] observed; */
    /* (int, int)[] stack; */
}

Expected output:

struct A {
    x: usize,
    /* protected int[] observed;
     * (int, int)[] stack; */
}

or

struct A {
    x: usize,
    // protected int[] observed;
    // (int, int)[] stack;
}

Actual output:

struct A {
    x: usize,
    /* protected int[] observed; */
     * (int, int)[] stack; */
}

Reproduce the example with rustfmt using normalize_comments = true, then trace the comment-normalization entry point responsible for adjacent block comments. The fix is done when the input produces one valid block comment or separate line comments without duplicated delimiters, with a regression test covering this case.