Preserve FieldMask as a message in the CelValue runtime path

BaseProtoCelValueConverter.fromWellKnownProto converts FieldMask to a
comma-separated string of its paths. That's correct in JSON-assignment
contexts (handled separately by CelProtoJsonAdapter), but it breaks CEL
expressions that access fields on the FieldMask itself — e.g.
`fieldMask.paths` — because the string has no field `paths`.

The CEL spec's WKT conversion table does not list FieldMask. cel-go
treats it as a regular message, as does cel-java's legacy
ProtoLiteAdapter.adaptValueToWellKnownProto for non-JSON contexts.
Override fromWellKnownProto in ProtoCelValueConverter to preserve
FieldMask as a ProtoMessageValue. JSON-assignment paths (e.g.
TestAllTypes{single_value: FieldMask{...}}) are unaffected because
EvalCreateStruct and CelValueRuntimeTypeProvider.createMessage both
unwrap StructValue results before the outer assignment runs, and the
outer assignment goes through CelProtoJsonAdapter.adaptValueToJsonValue
which handles FieldMask → string conversion at that layer.

Author: andrewparmet

common/src/main/java/dev/cel/common/values/ProtoCelValueConverter.java

@@ -71,6 +71,9 @@ protected Object fromWellKnownProto(MessageLiteOrBuilder msg, WellKnownProto wel
               "Unpacking failed for message: " + message.getDescriptorForType().getFullName(), e);
         }
         return toRuntimeValue(unpackedMessage);
+      case FIELD_MASK:
+        return ProtoMessageValue.create(
+            (Message) message, celDescriptorPool, this, celOptions.enableJsonFieldNames());
       default:
         return super.fromWellKnownProto(message, wellKnownProto);
     }

common/src/test/java/dev/cel/common/values/ProtoMessageValueTest.java

@@ -303,6 +303,24 @@ public void selectField_durationOutOfRange_success(int seconds, int nanos) {
         .isEqualTo(Duration.ofSeconds(seconds, nanos));
   }
 
+  @Test
+  public void selectField_fieldMask_returnsProtoMessageValue() {
+    TestAllTypes testAllTypes =
+        TestAllTypes.newBuilder()
+            .setFieldMask(
+                com.google.protobuf.FieldMask.newBuilder().addPaths("foo").addPaths("bar"))
+            .build();
+
+    ProtoMessageValue protoMessageValue =
+        ProtoMessageValue.create(
+            testAllTypes, DefaultDescriptorPool.INSTANCE, PROTO_CEL_VALUE_CONVERTER, false);
+
+    Object selected = protoMessageValue.select("field_mask");
+    assertThat(selected).isInstanceOf(ProtoMessageValue.class);
+    assertThat(((ProtoMessageValue) selected).select("paths"))
+        .isEqualTo(ImmutableList.of("foo", "bar"));
+  }
+
   @SuppressWarnings("ImmutableEnumChecker") // Test only
   private enum SelectFieldJsonValueTestCase {
     NULL(Value.newBuilder().build(), NullValue.NULL_VALUE),