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/fixture_test.cc

@@ -34,6 +34,13 @@ TEST_F(MujocoTestTest, MjUserWarningFailsTest) {
   EXPECT_NONFATAL_FAILURE(mju_warning("Warning."), "Warning.");
 }
 
+TEST_F(MujocoTestTest, BenignWarningDoesNotFailTest) {
+  // Warnings in the benign list should not trigger test failures
+  mju_warning(
+      "flex 'soft' is not rigid and has no equality constraints "
+      "or passive forces");
+}
+
 TEST_F(MujocoTestTest, MjUserErrorFailsTest) {
   EXPECT_FATAL_FAILURE(mju_error("Error."), "Error.");
 }