Closes #5057

## Summary

The indent-based code fold detection does not account for lines inside
multiline strings or block comments that have less indentation than the
surrounding code. This causes the fold scanner to think the enclosing
block ends prematurely.

For example, folding `fn main()` here would fail because the raw string
content at indent level 0 gets treated as the end of the function body:

``` rust
fn main() {
    let s = r#"
unindented content
"#;
}  
```

This PR checks whether a low-indent line falls inside a string or
comment override region and skips it if so. This works across all
languages that define `@string` or `@comment` overrides in their
`overrides.scm`.

## Before


https://github.com/user-attachments/assets/a08e6bf8-4f25-4211-8a46-8f6da7e49247

## After


https://github.com/user-attachments/assets/cd5b36db-6d4d-420b-9d60-79f9fad8638e

## Test Plan

- Added `test_fold_with_unindented_multiline_raw_string`
- Added `test_fold_with_unindented_multiline_block_comment`
- All existing fold tests pass
- Manually tested both Rust and Python examples

Release Notes:

- Fixed code folding incorrectly collapsing when multiline strings or
block comments contained unindented content