Properly fix multi-mixin members in shape build

Shape builders have a saftey feature to fix members whose mixin list
doesn't match the expected list based on the container's mixin list.
This fixes a bug in that where it would drop all but the last mixin
from that "fixed" list.

smithy-model/src/main/java/software/amazon/smithy/model/shapes/NamedMemberUtils.java

@@ -53,7 +53,7 @@ static Map<String, MemberShape> computeMixinMembers(
                     // but omits the mixin members of parent shapes. Arguably, that's way too
                     // nuanced and error-prone to _not_ try to smooth over.
                     if (localMember.getMixins().isEmpty() || !localMember.getMixins().contains(member.getId())) {
-                        localMember = localMember.toBuilder().clearMixins().addMixin(member).build();
+                        localMember = rebuildMemberMixins(localMember, mixins.values());
                     }
                     computedMembers.put(name, localMember);
                 } else {
@@ -87,6 +87,14 @@ static Map<String, MemberShape> computeMixinMembers(
         return Collections.unmodifiableMap(computedMembers);
     }
 
+    private static MemberShape rebuildMemberMixins(MemberShape member, Collection<Shape> mixins) {
+        MemberShape.Builder builder = member.toBuilder().clearMixins();
+        for (Shape mixin : mixins) {
+            mixin.getMember(member.getMemberName()).ifPresent(builder::addMixin);
+        }
+        return builder.build();
+    }
+
     static Set<MemberShape> flattenMixins(
             Map<String, MemberShape> members,
             Map<ShapeId, Shape> mixins,

smithy-model/src/test/java/software/amazon/smithy/model/shapes/StructureShapeTest.java

@@ -310,4 +310,30 @@ public void redefiningMembersPreservesOrder() {
 
         assertThat(concrete.getMemberNames(), contains("a", "b", "c", "d"));
     }
+
+    @Test
+    public void fixesMissingMemberMixins() {
+        StringShape string = StringShape.builder().id("smithy.example#String").build();
+        StructureShape mixin1 = StructureShape.builder()
+                .id("smithy.example#Mixin1")
+                .addTrait(MixinTrait.builder().build())
+                .addMember("a", string.getId())
+                .build();
+        StructureShape mixin2 = StructureShape.builder()
+                .id("smithy.example#Mixin2")
+                .addTrait(MixinTrait.builder().build())
+                .addMember("a", string.getId())
+                .build();
+        StructureShape concrete = StructureShape.builder()
+                .id("smithy.example#Concrete")
+                .addMember("a", string.getId())
+                .addMixin(mixin1)
+                .addMixin(mixin2)
+                .build();
+
+        assertThat(concrete.getMember("a").get().getMixins(), contains(
+                ShapeId.from("smithy.example#Mixin1$a"),
+                ShapeId.from("smithy.example#Mixin2$a")
+        ));
+    }
 }