Escape route prefixes in active_for_routes before using them as regex

[javihgil]

Problem

active_for_routes() currently uses the provided prefix directly inside preg_match().

That means route prefixes containing regular expression characters can behave incorrectly or trigger unexpected matching behavior.

Current evidence

• src/Twig/Extension/ActiveForRoutesExtension.php builds the pattern with "/^$routesStartsWith/i"
• the input is treated as regex syntax instead of plain route text

Expected behavior

Route prefixes should be matched as route text, not as arbitrary regular expressions.

Suggested work

• escape the user-provided prefix before building the regex
• keep the current support for matching both the raw prefix and the admin_ prefixed variant
• add tests for prefixes containing regex-special characters