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

src/xml/xml_api.cc

@@ -58,8 +58,16 @@ mjModel* mj_loadXML(const char* filename, const mjVFS* vfs,
   }
 
   // handle compile warning
-  if (mjs_isWarning(spec.get())) {
-    mjCopyError(error, mjs_getError(spec.get()), error_sz);
+  int num_warnings = mjs_numWarnings(spec.get());
+  if (num_warnings > 0) {
+    std::string all_warnings;
+    for (int i = 0; i < num_warnings; ++i) {
+      if (!all_warnings.empty()) {
+        all_warnings += '\n';
+      }
+      all_warnings += mjs_getWarning(spec.get(), i);
+    }
+    mjCopyError(error, all_warnings.c_str(), error_sz);
   } else if (error) {
     error[0] = '\0';
   }