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/xml/xml_write_read_test.cc

@@ -84,6 +84,12 @@ class WriteReadCompareTest : public XMLWriterTest,
 TEST_P(WriteReadCompareTest, WriteReadCompare) {
   std::string xml = GetParam();
 
+  // If this is the flex_line_obj model, expect the 'is not rigid' warning
+  if (absl::StrContains(xml, "flex_line_obj")) {
+    EXPECT_CALL(mock_warning_handler, Warn(testing::HasSubstr("is not rigid")))
+        .WillRepeatedly(testing::Return());
+  }
+
   // full precision float printing
   FullFloatPrecision increase_precision;