[flake8-comprehensions] Fix C420 to prepend whitespace when needed (#18616)

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## Summary
This PR fixes rule C420's fix. The fix replaces `{...}` with
`dict....(...)`. Therefore, if there is any identifier or such right
before the fix, the fix will fuse that previous token with `dict...`.

The example in the issue is
```python
0 or{x: None for x in "x"}
# gets "fixed" to
0 ordict.fromkeys(iterable)
```

## Related Issues

Fixes: https://github.com/astral-sh/ruff/issues/18599

crates/ruff_linter/resources/test/fixtures/flake8_comprehensions/C420_2.py

@@ -0,0 +1,5 @@
+foo or{x: None for x in bar}
+
+
+# C420 fix must make sure to insert a leading space if needed,
+# See https://github.com/astral-sh/ruff/issues/18599

crates/ruff_linter/src/rules/flake8_comprehensions/mod.rs

@@ -24,6 +24,7 @@ mod tests {
     #[test_case(Rule::UnnecessaryComprehensionInCall, Path::new("C419_2.py"))]
     #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420.py"))]
     #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420_1.py"))]
+    #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420_2.py"))]
     #[test_case(Rule::UnnecessaryDoubleCastOrProcess, Path::new("C414.py"))]
     #[test_case(Rule::UnnecessaryGeneratorDict, Path::new("C402.py"))]
     #[test_case(Rule::UnnecessaryGeneratorList, Path::new("C400.py"))]

crates/ruff_linter/src/rules/flake8_comprehensions/rules/unnecessary_dict_comprehension_for_iterable.rs

@@ -7,6 +7,7 @@ use ruff_python_ast::{self as ast, Arguments, Comprehension, Expr, ExprCall, Exp
 use ruff_text_size::{Ranged, TextRange};
 
 use crate::checkers::ast::Checker;
+use crate::fix::edits::pad_start;
 use crate::{Edit, Fix, FixAvailability, Violation};
 
 /// ## What it does
@@ -136,12 +137,16 @@ pub(crate) fn unnecessary_dict_comprehension_for_iterable(
 
     if checker.semantic().has_builtin_binding("dict") {
         let edit = Edit::range_replacement(
-            checker
-                .generator()
-                .expr(&fix_unnecessary_dict_comprehension(
-                    dict_comp.value.as_ref(),
-                    generator,
-                )),
+            pad_start(
+                checker
+                    .generator()
+                    .expr(&fix_unnecessary_dict_comprehension(
+                        dict_comp.value.as_ref(),
+                        generator,
+                    )),
+                dict_comp.start(),
+                checker.locator(),
+            ),
             dict_comp.range(),
         );
         diagnostic.set_fix(Fix::applicable_edit(

crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C420_C420_2.py.snap

@@ -0,0 +1,16 @@
+---
+source: crates/ruff_linter/src/rules/flake8_comprehensions/mod.rs
+---
+C420_2.py:1:7: C420 [*] Unnecessary dict comprehension for iterable; use `dict.fromkeys` instead
+  |
+1 | foo or{x: None for x in bar}
+  |       ^^^^^^^^^^^^^^^^^^^^^^ C420
+  |
+  = help: Replace with `dict.fromkeys(iterable, value)`)
+
+ℹ Safe fix
+1   |-foo or{x: None for x in bar}
+  1 |+foo or dict.fromkeys(bar)
+2 2 | 
+3 3 | 
+4 4 | # C420 fix must make sure to insert a leading space if needed,