[ruff] Fix syntax error introduced for an empty string followed by a u-prefixed string (UP025) (#18899)

## Summary
/closes #18895
## Test Plan

---------

Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP025.py

@@ -26,3 +26,9 @@ def hello():
 
 f"foo"u"bar"  # OK
 f"foo" u"bar"  # OK
+
+# https://github.com/astral-sh/ruff/issues/18895
+""u""
+""u"hi"
+""""""""""""""""""""u"hi"
+""U"helloooo"

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

@@ -42,9 +42,28 @@ impl AlwaysFixableViolation for UnicodeKindPrefix {
 pub(crate) fn unicode_kind_prefix(checker: &Checker, string: &StringLiteral) {
     if string.flags.prefix().is_unicode() {
         let mut diagnostic = checker.report_diagnostic(UnicodeKindPrefix, string.range);
-        diagnostic.set_fix(Fix::safe_edit(Edit::range_deletion(TextRange::at(
-            string.start(),
-            TextSize::from(1),
-        ))));
+
+        let prefix_range = TextRange::at(string.start(), TextSize::new(1));
+        let locator = checker.locator();
+        let content = locator
+            .slice(TextRange::new(prefix_range.end(), string.end()))
+            .to_owned();
+
+        // If the preceding character is equivalent to the quote character, insert a space to avoid a
+        // syntax error. For example, when removing the `u` prefix in `""u""`, rewrite to `"" ""`
+        // instead of `""""`.
+        // see https://github.com/astral-sh/ruff/issues/18895
+        let edit = if locator
+            .slice(TextRange::up_to(prefix_range.start()))
+            .chars()
+            .last()
+            .is_some_and(|char| content.starts_with(char))
+        {
+            Edit::range_replacement(" ".to_string(), prefix_range)
+        } else {
+            Edit::range_deletion(prefix_range)
+        };
+
+        diagnostic.set_fix(Fix::safe_edit(edit));
     }
 }

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP025.py.snap

@@ -283,12 +283,16 @@ UP025.py:27:7: UP025 [*] Remove unicode literals from strings
 27    |-f"foo"u"bar"  # OK
    27 |+f"foo" "bar"  # OK
 28 28 | f"foo" u"bar"  # OK
+29 29 | 
+30 30 | # https://github.com/astral-sh/ruff/issues/18895
 
 UP025.py:28:8: UP025 [*] Remove unicode literals from strings
    |
 27 | f"foo"u"bar"  # OK
 28 | f"foo" u"bar"  # OK
    |        ^^^^^^ UP025
+29 |
+30 | # https://github.com/astral-sh/ruff/issues/18895
    |
    = help: Remove unicode prefix
 
@@ -298,3 +302,80 @@ UP025.py:28:8: UP025 [*] Remove unicode literals from strings
 27 27 | f"foo"u"bar"  # OK
 28    |-f"foo" u"bar"  # OK
    28 |+f"foo" "bar"  # OK
+29 29 | 
+30 30 | # https://github.com/astral-sh/ruff/issues/18895
+31 31 | ""u""
+
+UP025.py:31:3: UP025 [*] Remove unicode literals from strings
+   |
+30 | # https://github.com/astral-sh/ruff/issues/18895
+31 | ""u""
+   |   ^^^ UP025
+32 | ""u"hi"
+33 | """"""""""""""""""""u"hi"
+   |
+   = help: Remove unicode prefix
+
+ℹ Safe fix
+28 28 | f"foo" u"bar"  # OK
+29 29 | 
+30 30 | # https://github.com/astral-sh/ruff/issues/18895
+31    |-""u""
+   31 |+"" ""
+32 32 | ""u"hi"
+33 33 | """"""""""""""""""""u"hi"
+34 34 | ""U"helloooo"
+
+UP025.py:32:3: UP025 [*] Remove unicode literals from strings
+   |
+30 | # https://github.com/astral-sh/ruff/issues/18895
+31 | ""u""
+32 | ""u"hi"
+   |   ^^^^^ UP025
+33 | """"""""""""""""""""u"hi"
+34 | ""U"helloooo"
+   |
+   = help: Remove unicode prefix
+
+ℹ Safe fix
+29 29 | 
+30 30 | # https://github.com/astral-sh/ruff/issues/18895
+31 31 | ""u""
+32    |-""u"hi"
+   32 |+"" "hi"
+33 33 | """"""""""""""""""""u"hi"
+34 34 | ""U"helloooo"
+
+UP025.py:33:21: UP025 [*] Remove unicode literals from strings
+   |
+31 | ""u""
+32 | ""u"hi"
+33 | """"""""""""""""""""u"hi"
+   |                     ^^^^^ UP025
+34 | ""U"helloooo"
+   |
+   = help: Remove unicode prefix
+
+ℹ Safe fix
+30 30 | # https://github.com/astral-sh/ruff/issues/18895
+31 31 | ""u""
+32 32 | ""u"hi"
+33    |-""""""""""""""""""""u"hi"
+   33 |+"""""""""""""""""""" "hi"
+34 34 | ""U"helloooo"
+
+UP025.py:34:3: UP025 [*] Remove unicode literals from strings
+   |
+32 | ""u"hi"
+33 | """"""""""""""""""""u"hi"
+34 | ""U"helloooo"
+   |   ^^^^^^^^^^^ UP025
+   |
+   = help: Remove unicode prefix
+
+ℹ Safe fix
+31 31 | ""u""
+32 32 | ""u"hi"
+33 33 | """"""""""""""""""""u"hi"
+34    |-""U"helloooo"
+   34 |+"" "helloooo"