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: I4b580d1e811428085db7d8d43c36a72ca788ad5b

Author: yuvaltassa

mjpc/test/load.cc

@@ -42,6 +42,7 @@ mjModel* LoadTestModel(std::string_view path) {
   mjModel* model = mj_loadXML(filename, nullptr, loadError, 1000);
   if (loadError[0]) std::cerr << "load error: " << loadError << '\n';
 
+  mj_freeLastXML();
   return model;
 }