Avoid detecting continuations at non-start-of-line (#6219)

## Summary

Previously, given:

```python
a = \
  5;
```

When detecting continuations starting at the offset of the `;`, we'd
flag the previous line as a continuation. We should only flag a
continuation if there isn't leading content prior to the offset.

Closes https://github.com/astral-sh/ruff/issues/6214

crates/ruff/resources/test/fixtures/pycodestyle/E70.py

@@ -66,3 +66,6 @@ while 1:
 #: E703:2:1
 0\
 ;
+#: E701:2:3
+a = \
+  5;

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__E703_E70.py.snap

@@ -70,6 +70,8 @@ E70.py:68:1: E703 [*] Statement ends with an unnecessary semicolon
 67 | 0\
 68 | ;
    | ^ E703
+69 | #: E701:2:3
+70 | a = \
    |
    = help: Remove unnecessary semicolon
 
@@ -80,5 +82,24 @@ E70.py:68:1: E703 [*] Statement ends with an unnecessary semicolon
 67    |-0\
 68    |-;
    67 |+0
+69 68 | #: E701:2:3
+70 69 | a = \
+71 70 |   5;
+
+E70.py:71:4: E703 [*] Statement ends with an unnecessary semicolon
+   |
+69 | #: E701:2:3
+70 | a = \
+71 |   5;
+   |    ^ E703
+   |
+   = help: Remove unnecessary semicolon
+
+ℹ Fix
+68 68 | ;
+69 69 | #: E701:2:3
+70 70 | a = \
+71    |-  5;
+   71 |+  5
 
 

crates/ruff_python_index/src/indexer.rs

@@ -228,8 +228,18 @@ impl Indexer {
         offset: TextSize,
         locator: &Locator,
     ) -> Option<TextSize> {
-        // Find the first preceding continuation.
-        let mut continuation = self.find_continuation(locator.line_start(offset), locator)?;
+        // Find the first preceding continuation. If the offset isn't the first non-whitespace
+        // character on the line, then we can't have a continuation.
+        let previous_line_end = locator.line_start(offset);
+        if !locator
+            .slice(TextRange::new(previous_line_end, offset))
+            .chars()
+            .all(is_python_whitespace)
+        {
+            return None;
+        }
+
+        let mut continuation = self.find_continuation(previous_line_end, locator)?;
 
         // Continue searching for continuations, in the unlikely event that we have multiple
         // continuations in a row.