More schema-related cleanup.

- Declare every nonzero default; make the defaults cross-check total.
- Skip default-valued attributes in the hand-written writer paths.
- Fix type facts on hand-read elements, found by the dm_control diff.

PiperOrigin-RevId: 958999733
Change-Id: I3064ccc6ae1f049c20f273abc234cd02990a8b7e

test/xml/xml_api_test.cc

@@ -135,8 +135,8 @@ TEST_F(MujocoTest, SaveXmlShortString) {
   std::array<char, 10> out;
   EXPECT_THAT(mj_saveXMLString(spec, out.data(), out.size(), error.data(),
                                error.size()),
-              273);
-  EXPECT_STREQ(error.data(), "Output string too short, should be at least 274");
+              223);
+  EXPECT_STREQ(error.data(), "Output string too short, should be at least 224");
 
   mj_deleteSpec(spec);
   mj_deleteModel(model);