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_sensor_test.cc

@@ -1753,6 +1753,8 @@ TEST_F(SensorTest, InsideSiteFlexBody) {
   )";
 
   char error[1024] = {0};
+  EXPECT_CALL(mock_warning_handler, Warn(testing::HasSubstr("is not rigid")))
+      .WillOnce(testing::Return());
   mjModel* m = LoadModelFromString(xml, error, sizeof(error));
   ASSERT_THAT(m, NotNull()) << error;
   mjData* d = mj_makeData(m);
@@ -1839,6 +1841,8 @@ TEST_F(SensorTest, FlexContactSensors) {
   )";
 
   char error[1024] = {0};
+  EXPECT_CALL(mock_warning_handler, Warn(testing::HasSubstr("is not rigid")))
+      .WillOnce(testing::Return());
   mjModel* m = LoadModelFromString(xml, error, sizeof(error));
   ASSERT_THAT(m, NotNull()) << error;
   mjData* d = mj_makeData(m);