Fix duplicate error_message symbol

[Kyle-Ye]

Summary

• Make error_message static in assert.cpp to use internal linkage, avoiding linker collision when both OpenRenderBox and OpenAttributeGraph are linked together

Test plan

• Build passes when linked alongside OpenAttributeGraph

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (6dce9ce) to head (b21a3e2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #22   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          4       4           
  Lines        123     123           
=====================================
  Misses       123     123           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.