Add policies for global attribute conflict resolution upon attach

For example when loading parent_merge.xml:

```
WARNING: Attach conflict when attaching 'child' to 'parent_merge', policy is 'merge'
timestep: parent has 0.005, child has 0.002, taking the minimum
iterations: parent has 50, child has 100, taking the maximum
flag 'Damper': added from child
```

When loading parent_error.xml:

```
XML Error: Attach conflict when attaching 'child' to 'parent_error', policy is 'error'
timestep: parent has 0.005, child has 0.002
iterations: parent has 50, child has 100
Element 'attach', line 10
```
PiperOrigin-RevId: 933620810
Change-Id: Ib477863b5ef763474d27fb4be5a4148be1d5d500

test/xml/mjz/mjz_encoder_test.cc

@@ -99,6 +99,8 @@ std::vector<std::string> GetWriteReadTestModels() {
             // exclude files that fail since we do not save pinned flex nodes
             absl::StrContains(xml, "gripper_trilinear") ||
             absl::StrContains(xml, "strain") ||
+            // exclude conflict test assets (designed to fail compile)
+            absl::StrContains(xml, "xml/testdata/parent_") ||
             // exclude mjz testdata with VFS files
             absl::StrContains(xml, "mixed_test")) {
           continue;