C#: Add extension method flag to type model (#7075)

Set bit 20 in CSharpTypeMapping.MapFlags when IMethodSymbol.IsExtensionMethod
is true. This bit is unused by both the JVM spec and Java's Flag enum.

On the Java side, widen the Method constructor's flag mask to preserve bit 20
and add JavaType.Method.isExtensionMethod() so recipes can detect extension
methods without knowing the raw bit position.

Author: knutwannheden

rewrite-csharp/csharp/OpenRewrite/CSharp/CSharpTypeMapping.cs

@@ -351,6 +351,8 @@ private static long MapFlags(ISymbol symbol)
         if (symbol.IsStatic) flags |= 8;     // Flag.Static
         if (symbol.IsAbstract) flags |= 1024; // Flag.Abstract
         if (symbol.IsSealed) flags |= 16;     // Flag.Final (sealed ~ final)
+        // C#-specific: mark extension methods (bit 20, not used by Java Flag enum)
+        if (symbol is IMethodSymbol { IsExtensionMethod: true }) flags |= 1L << 20;
         return flags;
     }
 

rewrite-java/src/main/java/org/openrewrite/java/tree/JavaType.java

@@ -1296,6 +1296,10 @@ public boolean isNumeric() {
     @Getter
     @FieldDefaults(makeFinal = true, level = AccessLevel.PRIVATE)
     class Method implements JavaType {
+        // C#-specific: marks extension methods (bit 20, not used by Java Flag enum)
+        private static final long EXTENSION_METHOD_FLAG = 1L << 20;
+        private static final long VALID_METHOD_FLAGS = Flag.VALID_FLAGS | EXTENSION_METHOD_FLAG;
+
         @With
         @Nullable
         @NonFinal
@@ -1387,7 +1391,7 @@ public Method(@Nullable Integer managedReference, long flagsBitMap, @Nullable Fu
                       FullyQualified @Nullable [] annotations, @Nullable List<String> defaultValue,
                       String @Nullable [] declaredFormalTypeNames) {
             this.managedReference = managedReference;
-            this.flagsBitMap = flagsBitMap & Flag.VALID_FLAGS;
+            this.flagsBitMap = flagsBitMap & VALID_METHOD_FLAGS;
             this.declaringType = unknownIfNull(declaringType);
             this.name = name;
             this.returnType = unknownIfNull(returnType);
@@ -1636,6 +1640,13 @@ public Method withFlags(Set<Flag> flags) {
             return withFlagsBitMap(Flag.flagsToBitMap(flags));
         }
 
+        /**
+         * @return {@code true} if this method is a C# extension method.
+         */
+        public boolean isExtensionMethod() {
+            return (flagsBitMap & EXTENSION_METHOD_FLAG) != 0;
+        }
+
         @Override
         public boolean equals(Object o) {
             if (this == o) return true;