[pyupgrade]: Improve diagnostic range for redundant-open-mode (UP015) (#16672)

## Summary

This PR stabilizes the behavior change introduced in
#15872

The diagnostic range is now the range of the redundant `mode` argument
where it previously was the range of the entire `open` call:

Before:

```
UP015.py:2:1: UP015 [*] Unnecessary mode argument
  |
1 | open("foo", "U")
2 | open("foo", "Ur")
  | ^^^^^^^^^^^^^^^^^ UP015
3 | open("foo", "Ub")
4 | open("foo", "rUb")
  |
  = help: Remove mode argument
```


Now:

```
UP015.py:2:13: UP015 [*] Unnecessary mode argument
  |
1 | open("foo", "U")
2 | open("foo", "Ur")
  |             ^^^^ UP015
3 | open("foo", "Ub")
4 | open("foo", "rUb")
  |
  = help: Remove mode argument
```

This is a breaking change because it may require moving a `noqa` comment
onto a different line, e.g if you have

```py
open(
    "foo",
    "Ur",
) # noqa: UP015
```

Needs to be rewritten to 

```py
open(
    "foo",
    "Ur", # noqa: UP015
)
```

There have been now new issues or PRs since the new preview behavior was
implemented. It first was released as part of Ruff 0.9.5 on the 5th of
Feb (a little more than a month ago)

## Test Plan

I reviewed the snapshot tests

Author: MichaReiser

crates/ruff_linter/src/rules/pyupgrade/mod.rs

@@ -119,25 +119,6 @@ mod tests {
         Ok(())
     }
 
-    #[test_case(Rule::RedundantOpenModes, Path::new("UP015.py"))]
-    #[test_case(Rule::RedundantOpenModes, Path::new("UP015_1.py"))]
-    fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
-        let snapshot = format!(
-            "preview__{}_{}",
-            rule_code.noqa_code(),
-            path.to_string_lossy()
-        );
-        let diagnostics = test_path(
-            Path::new("pyupgrade").join(path).as_path(),
-            &settings::LinterSettings {
-                preview: PreviewMode::Enabled,
-                ..settings::LinterSettings::for_rule(rule_code)
-            },
-        )?;
-        assert_messages!(snapshot, diagnostics);
-        Ok(())
-    }
-
     #[test]
     fn up007_preview() -> Result<()> {
         let diagnostics = test_path(

crates/ruff_linter/src/rules/pyupgrade/rules/redundant_open_modes.rs

@@ -91,17 +91,11 @@ fn create_diagnostic(
     mode: OpenMode,
     checker: &Checker,
 ) -> Diagnostic {
-    let range = if checker.settings.preview.is_enabled() {
-        mode_arg.range()
-    } else {
-        call.range
-    };
-
     let mut diagnostic = Diagnostic::new(
         RedundantOpenModes {
             replacement: mode.to_string(),
         },
-        range,
+        mode_arg.range(),
     );
 
     if mode.is_empty() {