Fix a PHP 8.5 chr() deprecation when decoding octal string escapes

PHP 8.5 deprecates passing a value outside the [0, 255] range to chr().
The string-escape decoder in the lexer accepts up to three octal digits,
so an escape such as "\777" (= 511) reaches chr() out of range and emits:

    chr(): Providing a value not in-between 0 and 255 is deprecated ...

chr() already constrains the value with "% 256", so applying "% 256"
explicitly preserves the exact byte while silencing the deprecation. The
hex escape branch is unaffected because it is capped at two digits (0xff).

CHANGELOG

@@ -1,5 +1,6 @@
 # 3.28.0 (2026-XX-XX)
 
+ * Fix a PHP 8.5 `chr()` deprecation when decoding an octal string escape sequence larger than `\377` (such as `"\777"`)
  * Mark `Twig\Markup` as `@final`; it will be final in Twig 4.0
  * Reduce memory usage and speed up the context restoration compiled at the end of `for` loops
  * Allow calling a macro with a dynamic name via the dot operator (`macros.(name)(args)`)

src/Lexer.php

@@ -435,7 +435,7 @@ class Lexer
                 while ($i + 1 < $length && ctype_digit($str[$i + 1]) && $str[$i + 1] < '8' && \strlen($octal) < 3) {
                     $octal .= $str[++$i];
                 }
-                $result .= \chr(octdec($octal));
+                $result .= \chr(octdec($octal) % 256);
             } else {
                 trigger_deprecation('twig/twig', '3.12', 'Character "%s" should not be escaped; the "\" character is ignored in Twig 3 but will not be in Twig 4. Please remove the extra "\" character at position %d in "%s" at line %d.', $nextChar, $i + 1, $this->source->getName(), $this->lineno);
                 $result .= $nextChar;

tests/LexerTest.php

@@ -209,6 +209,18 @@ class LexerTest extends TestCase
             EOF,
             "\065\x64",
         ];
+        yield 'octal escape overflowing a single byte is constrained with % 256' => [
+            <<<'EOF'
+            {{ '\777' }}
+            EOF,
+            "\xff",
+        ];
+        yield 'octal escape exactly equal to 256 wraps around to a NUL byte' => [
+            <<<'EOF'
+            {{ '\400' }}
+            EOF,
+            "\x00",
+        ];
         yield [
             <<<'EOF'
             {{ 'App\\Test' }}