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/engine/engine_forward_test.cc

@@ -80,7 +80,9 @@ struct ActLimitedTestCase {
   mjtIntegrator integrator;
 };
 
-using ParametrizedForwardTest = ::testing::TestWithParam<ActLimitedTestCase>;
+class ParametrizedForwardTest
+    : public MujocoTest,
+      public ::testing::WithParamInterface<ActLimitedTestCase> {};
 
 TEST_P(ParametrizedForwardTest, ActLimited) {
   static constexpr char xml[] = R"(