Fallback to end-of-file if ends in trailing continuation (#6789)

## Summary

Given:

```python
def end_of_file():
    if False:
        return 1
    x = 2 \

```

Then when searching for the end of the `x = 2` statement, we'd reach a
panic as we'd hit the last line (`\\`) and abort, since the universal
iterator doesn't return trailing newlines. Instead, we should just use
the end of the file as the fallback.

Closes https://github.com/astral-sh/ruff/issues/6787.

## Test Plan

`cargo test`

crates/ruff/resources/test/fixtures/flake8_return/RET503.py

@@ -320,3 +320,9 @@ def end_of_statement():
         if True:
             return "" \
                 ;  # type: ignore
+
+
+def end_of_file():
+    if False:
+        return 1
+    x = 2 \

crates/ruff/src/rules/flake8_return/helpers.rs

@@ -25,19 +25,11 @@ pub(super) fn result_exists(returns: &[&ast::StmtReturn]) -> bool {
 /// This method assumes that the statement is the last statement in its body; specifically, that
 /// the statement isn't followed by a semicolon, followed by a multi-line statement.
 pub(super) fn end_of_last_statement(stmt: &Stmt, locator: &Locator) -> TextSize {
-    if stmt.end() == locator.text_len() {
-        // End-of-file, so just return the end of the statement.
-        stmt.end()
-    } else {
-        // Otherwise, find the end of the last line that's "part of" the statement.
-        let contents = locator.after(stmt.end());
-
-        for line in contents.universal_newlines() {
-            if !line.ends_with('\\') {
-                return stmt.end() + line.end();
-            }
+    // Find the end of the last line that's "part of" the statement.
+    for line in locator.after(stmt.end()).universal_newlines() {
+        if !line.ends_with('\\') {
+            return stmt.end() + line.end();
         }
-
-        unreachable!("Expected to find end-of-statement")
     }
+    locator.text_len()
 }

crates/ruff/src/rules/flake8_return/snapshots/ruff__rules__flake8_return__tests__RET503_RET503.py.snap

@@ -380,5 +380,24 @@ RET503.py:320:9: RET503 [*] Missing explicit `return` at the end of function abl
 321 321 |             return "" \
 322 322 |                 ;  # type: ignore
     323 |+        return None
+323 324 | 
+324 325 | 
+325 326 | def end_of_file():
+
+RET503.py:328:5: RET503 [*] Missing explicit `return` at the end of function able to return non-`None` value
+    |
+326 |     if False:
+327 |         return 1
+328 |     x = 2 \
+    |     ^^^^^ RET503
+    |
+    = help: Add explicit `return` statement
+
+ℹ Suggested fix
+326 326 |     if False:
+327 327 |         return 1
+328 328 |     x = 2 \
+    329 |+
+    330 |+    return None