[DO NOT MERGE] Temporarily force PR builds to test 3.15/3.15t on macOS

PR builds normally limit to python_versions[0] (3.10); override that to
["3.15", "3.15t"] for macOS arm64 so this PR's CI actually exercises the new
versions. Other OSes and callers that pass an explicit python-versions list
are unaffected. Revert this commit before merging.

Authored with assistance from Claude Code (AI assistant).

Author: atalman

tools/scripts/generate_binary_build_matrix.py

@@ -494,7 +494,11 @@ def generate_wheels_matrix(
             arches += [XPU]
 
     if limit_pr_builds:
-        python_versions = [python_versions[0]]
+        # TEMPORARY (DO NOT MERGE): revert before merging.
+        if not explicitly_requested_versions and os == MACOS_ARM64:
+            python_versions = ["3.15", "3.15t"]
+        else:
+            python_versions = [python_versions[0]]
 
     global WHEEL_CONTAINER_IMAGES