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

python/mujoco/specs_test.py

@@ -597,6 +597,18 @@ class SpecsTest(absltest.TestCase):
     with self.assertRaisesRegex(ValueError, expected_error):
       spec.compile()
 
+  def test_compile_warnings(self):
+    xml = """
+    <mujoco>
+      <worldbody>
+        <flexcomp name="my_flex" type="grid" count="3 3 1" spacing=".05 .05 .05" radius=".01" dim="2"/>
+      </worldbody>
+    </mujoco>
+    """
+    spec = mujoco.MjSpec.from_string(xml)
+    with self.assertWarnsRegex(UserWarning, 'is not rigid'):
+      spec.compile()
+
   def test_recompile(self):
     # Create a spec.
     spec = mujoco.MjSpec()