Refactor compiler warning handling.

Compiler warnings are now accumulated in a vector of strings within the mjSpec object. New API functions `mjs_numWarnings` and `mjs_getWarning` are added to access these warnings. The compiler's log handler now chains warnings to the global log handler, ensuring they are still displayed immediately. Call sites in `mj_loadXML`, `mj_compile`, and the Python and WASM bindings have been updated to use the new warning API.

PiperOrigin-RevId: 933361650
Change-Id: I47cab98a460c57b0898c0a1a43fce2a5b9648eb1

test/user/user_objects_test.cc

@@ -1181,7 +1181,7 @@ TEST_F(MjCGeomTest, IgnoreBadGeomOutsideInertiagrouprange) {
 TEST_F(MjCGeomTest, NanSize) {
   // even if the caller ignores warnings, models shouldn't compile with NaN
   // geom sizes
-  mju_user_warning = nullptr;
+  mock_warning_handler.ExpectWarnings();
   static constexpr char xml[] = R"(
   <mujoco>
     <worldbody>