Ignore end-of-line file exemption comments (#6160)

## Summary

This PR protects against code like:

```python
from typing import Optional

import bar  # ruff: noqa
import baz

class Foo:
    x: Optional[str] = None
```

In which the user wrote `# ruff: noqa` to ignore a specific error, not
realizing that it was a file-level exemption that thus turned off all
lint rules.

Specifically, if a `# ruff: noqa` directive is not at the start of a
line, we now ignore it and warn, since this is almost certainly a
mistake.

crates/ruff_python_trivia/src/whitespace.rs

@@ -2,7 +2,7 @@ use ruff_source_file::Locator;
 use ruff_text_size::{TextRange, TextSize};
 
 /// Extract the leading indentation from a line.
-pub fn indentation_at_offset<'a>(locator: &'a Locator, offset: TextSize) -> Option<&'a str> {
+pub fn indentation_at_offset<'a>(offset: TextSize, locator: &'a Locator) -> Option<&'a str> {
     let line_start = locator.line_start(offset);
     let indentation = &locator.contents()[TextRange::new(line_start, offset)];