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

doc/APIreference/functions.rst

@@ -2161,7 +2161,25 @@ Get compiler timing diagnostics from spec, returns pointer to array of size mjNC
 
 .. mujoco-include:: mjs_isWarning
 
-Return 1 if compiler error is a warning.
+Return 1 if compiler error is a warning. Deprecated: use mjs_numWarnings(s) > 0.
+
+.. _mjs_numWarnings:
+
+`mjs_numWarnings <#mjs_numWarnings>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_numWarnings
+
+Get number of warnings accumulated in the spec.
+
+.. _mjs_getWarning:
+
+`mjs_getWarning <#mjs_getWarning>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_getWarning
+
+Get the i-th warning message (returns nullptr if index out of bounds).
 
 .. _Miscellaneous: