Perform Reactor native classpath checks at build time

See gh-28624

spring-core/graalvm/src/main/java/org/springframework/aot/graalvm/ConstantFieldSubstitutionProcessor.java

@@ -45,7 +45,8 @@ class ConstantFieldSubstitutionProcessor extends SubstitutionProcessor {
 	private static Pattern[] patterns = {
 			Pattern.compile(Pattern.quote("org.springframework.core.NativeDetector#imageCode")),
 			Pattern.compile(Pattern.quote("org.springframework.") + ".*#.*Present"),
-			Pattern.compile(Pattern.quote("org.springframework.") + ".*#.*_PRESENT")
+			Pattern.compile(Pattern.quote("org.springframework.") + ".*#.*_PRESENT"),
+			Pattern.compile(Pattern.quote("reactor.") + ".*#.*Available")
 	};
 
 	private final ThrowawayClassLoader throwawayClassLoader;