Pass UnionShape for union type discrimination (#3984)

UnionShape needs to be passed so that the customization code can detect
which Union type is being generated.

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/CborParserGenerator.kt

@@ -78,8 +78,10 @@ abstract class CborParserCustomization : NamedCustomization<CborParserSection>()
      * @param defaultContext The default discrimination context containing decoder symbol and discriminator method.
      * @return UnionVariantDiscriminatorContext that defines how to discriminate union variants.
      */
-    open fun getUnionVariantDiscriminator(defaultContext: CborParserGenerator.UnionVariantDiscriminatorContext) =
-        defaultContext
+    open fun getUnionVariantDiscriminator(
+        unionShape: UnionShape,
+        defaultContext: CborParserGenerator.UnionVariantDiscriminatorContext,
+    ) = defaultContext
 }
 
 class CborParserGenerator(
@@ -330,7 +332,7 @@ class CborParserGenerator(
             val returnSymbolToParse = returnSymbolToParse(shape)
             // Get actual decoder type to use and the discriminating function to call to extract
             // the variant of the union that has been encoded in the data.
-            val discriminatorContext = getUnionDiscriminatorContext("Decoder", "decoder.str()?.as_ref()")
+            val discriminatorContext = getUnionDiscriminatorContext(shape, "Decoder", "decoder.str()?.as_ref()")
 
             rustBlockTemplate(
                 """
@@ -394,6 +396,7 @@ class CborParserGenerator(
         }
 
     private fun getUnionDiscriminatorContext(
+        unionShape: UnionShape,
         decoderType: String,
         callMethod: String,
     ): UnionVariantDiscriminatorContext {
@@ -403,7 +406,7 @@ class CborParserGenerator(
                 writable { rustTemplate(callMethod) },
             )
         return customizations.fold(defaultUnionPairContext) { context, customization ->
-            customization.getUnionVariantDiscriminator(context)
+            customization.getUnionVariantDiscriminator(unionShape, context)
         }
     }