Fix corner case involving terminal backslash after fixing W293 (#5172)

## Summary

Fixes #4404. 

Consider this file:
```python
if True:
    x = 1; \
<space><space><space>
```

The current implementation of W293 removes the 3 spaces on line 2. This
fix changes the file to:
```python
if True:
    x = 1; \
```
A file can't end in a `\`, according to Python's [lexical
analysis](https://docs.python.org/3/reference/lexical_analysis.html), so
subsequent iterations of the autofixer fail (the AST-based ones
specifically, since they depend on a valid syntax tree and get
re-parsed).

This patch examines the line before the line checked in `W293`. If its
first non-whitespace character is a `\`, the patch will extend the
diagnostic's fix range to all whitespace up until the previous line's
*second* non-whitespace character; that is, it deletes all spaces and
potential `\`s up until the next non-whitespace character on the
previous line.

## Test Plan
Ran `cargo run -p ruff_cli -- ~/Downloads/aa.py --fix --select W293,D100
--no-cache` against the above file. This resulted in:
```
/Users/evan/Downloads/aa.py:1:1: D100 Missing docstring in public module
Found 2 errors (1 fixed, 1 remaining).
```
The file's contents, after the fix:
```python
if True:
    x = 1;<space>
```
The `\` was removed, leaving the terminal space. The space should be
handled by `Rule::TrailingWhitespace`, not `BlankLineWithWhitespace`.

crates/ruff/src/checkers/physical_lines.rs

@@ -1,7 +1,7 @@
 //! Lint rules based on checking physical lines.
+use std::path::Path;
 
 use ruff_text_size::TextSize;
-use std::path::Path;
 
 use ruff_diagnostics::Diagnostic;
 use ruff_python_ast::source_code::{Indexer, Locator, Stylist};
@@ -146,7 +146,7 @@ pub(crate) fn check_physical_lines(
         }
 
         if enforce_trailing_whitespace || enforce_blank_line_contains_whitespace {
-            if let Some(diagnostic) = trailing_whitespace(&line, settings) {
+            if let Some(diagnostic) = trailing_whitespace(&line, locator, indexer, settings) {
                 diagnostics.push(diagnostic);
             }
         }
@@ -185,9 +185,10 @@ pub(crate) fn check_physical_lines(
 
 #[cfg(test)]
 mod tests {
+    use std::path::Path;
+
     use rustpython_parser::lexer::lex;
     use rustpython_parser::Mode;
-    use std::path::Path;
 
     use ruff_python_ast::source_code::{Indexer, Locator, Stylist};
 

crates/ruff/src/rules/pycodestyle/rules/trailing_whitespace.rs

@@ -2,6 +2,8 @@ use ruff_text_size::{TextLen, TextRange, TextSize};
 
 use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Edit, Fix};
 use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::helpers;
+use ruff_python_ast::source_code::{Indexer, Locator};
 use ruff_python_whitespace::Line;
 
 use crate::registry::Rule;
@@ -72,7 +74,12 @@ impl AlwaysAutofixableViolation for BlankLineWithWhitespace {
 }
 
 /// W291, W293
-pub(crate) fn trailing_whitespace(line: &Line, settings: &Settings) -> Option<Diagnostic> {
+pub(crate) fn trailing_whitespace(
+    line: &Line,
+    locator: &Locator,
+    indexer: &Indexer,
+    settings: &Settings,
+) -> Option<Diagnostic> {
     let whitespace_len: TextSize = line
         .chars()
         .rev()
@@ -86,16 +93,20 @@ pub(crate) fn trailing_whitespace(line: &Line, settings: &Settings) -> Option<Di
             if settings.rules.enabled(Rule::BlankLineWithWhitespace) {
                 let mut diagnostic = Diagnostic::new(BlankLineWithWhitespace, range);
                 if settings.rules.should_fix(Rule::BlankLineWithWhitespace) {
-                    #[allow(deprecated)]
-                    diagnostic.set_fix(Fix::unspecified(Edit::range_deletion(range)));
+                    // Remove any preceding continuations, to avoid introducing a potential
+                    // syntax error.
+                    diagnostic.set_fix(Fix::automatic(Edit::range_deletion(TextRange::new(
+                        helpers::preceded_by_continuations(line.start(), locator, indexer)
+                            .unwrap_or(range.start()),
+                        range.end(),
+                    ))));
                 }
                 return Some(diagnostic);
             }
         } else if settings.rules.enabled(Rule::TrailingWhitespace) {
             let mut diagnostic = Diagnostic::new(TrailingWhitespace, range);
             if settings.rules.should_fix(Rule::TrailingWhitespace) {
-                #[allow(deprecated)]
-                diagnostic.set_fix(Fix::unspecified(Edit::range_deletion(range)));
+                diagnostic.set_fix(Fix::automatic(Edit::range_deletion(range)));
             }
             return Some(diagnostic);
         }

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__W291_W29.py.snap

@@ -12,7 +12,7 @@ W29.py:4:6: W291 [*] Trailing whitespace
   |
   = help: Remove trailing whitespace
 
-ℹ Suggested fix
+ℹ Fix
 1 1 | #: Okay
 2 2 | # 情
 3 3 | #: W291:1:6
@@ -33,7 +33,7 @@ W29.py:11:35: W291 [*] Trailing whitespace
    |
    = help: Remove trailing whitespace
 
-ℹ Suggested fix
+ℹ Fix
 8  8  |     bang = 12
 9  9  | #: W291:2:35
 10 10 | '''multiline
@@ -54,7 +54,7 @@ W29.py:13:6: W291 [*] Trailing whitespace
    |
    = help: Remove trailing whitespace
 
-ℹ Suggested fix
+ℹ Fix
 10 10 | '''multiline
 11 11 | string with trailing whitespace'''   
 12 12 | #: W291 W292 noeol

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__W293_W29.py.snap

@@ -12,7 +12,7 @@ W29.py:7:1: W293 [*] Blank line contains whitespace
   |
   = help: Remove whitespace from blank line
 
-ℹ Suggested fix
+ℹ Fix
 4 4 | print 
 5 5 | #: W293:2:1
 6 6 | class Foo(object):