fix(extension): make isValidClassName regex non-backtracking

The class-name validation pattern in EmbeddedExtensionRepository used
ambiguous quantifiers where the inner character class allowed '$' and
the outer group matched '$'-prefixed segments. On input like 'A$A$A$A'
with a trailing mismatch, the engine explored exponentially many
partitions (CodeQL js/redos, alert #15).

Switch the three repetitions to possessive quantifiers (`*+`) so each
commit step is final. Semantics are preserved for all accepted and
rejected class names (verified against SecurityValidationTest).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jbachorik

btrace-extension/src/main/java/org/openjdk/btrace/extension/impl/EmbeddedExtensionRepository.java

@@ -281,7 +281,8 @@ static boolean isValidClassName(String className) {
     }
     // Basic validation: must be a valid Java identifier pattern
     // Allows: package.Class, package.Class$Inner
-    return className.matches("^[a-zA-Z_][a-zA-Z0-9_]*(\\.[a-zA-Z_][a-zA-Z0-9_$]*)*(\\$[a-zA-Z_][a-zA-Z0-9_$]*)*$");
+    return className.matches(
+        "^[a-zA-Z_][a-zA-Z0-9_]*+(\\.[a-zA-Z_][a-zA-Z0-9_$]*+)*+(\\$[a-zA-Z_][a-zA-Z0-9_$]*+)*+$");
   }
 
   /**