Replace exported C API error globals

[paulromano]

Description

Another PR focused on avoiding OPENMC_API for Windows support. This PR replaces the exported OPENMC_E_* variables with a C enum and replaces the fixed-size exported error-message buffer with an internal string exposed through a new openmc_get_err_msg C API function. C++ and Python callers now retrieve error text through the function instead of accessing shared-library data directly. This also has the nice effect of eliminating the fixed 256-character limit that was in place before.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[GuySten]

Looks good to me.