Prohibit line-based test generation for meaningless line types (comments, single brackets, etc)

We are not allowing line generation for empty lines, but it can be asked for comment lines (or brackets) if it is surrounded by class/function. The result will be almost identical to the whole class test generation, but it's weird.

In the following case, we cannot generate line-based test for `(*)` but we can do it for `(**)` (likely, because this comment is considered as part of a method):
```kotlin
// my comment here! - not allowed for line generation (*)

// my comment here! - allowed for line generation (**)
fun multwice(a: Int) = a * 2
```
**_Solution idea_**: For each line generation, check if it includes something besides comments, open/closed brackets and allow generation only in such case

Start by locating the line-based test generation entry point and reproduce the Kotlin example with comment-only and bracket-only lines. Trace how the selected line is classified, then verify that generation is rejected when a line contains only comments or brackets and remains allowed when it contains code such as the function declaration.

Written by the indexing model from the issue text.