Ensure photon cross sections are loaded when FileSource contains photons

[paulromano]

Description

If an OpenMC model uses a FileSource that has photons and/or electrons/positrons but settings.photon_transport hasn't been set to true, the model will run but segfault because photon cross sections were never loaded. This PR fixes that and adds a test to confirm that it works as intended.

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.

[shimwell]

thanks for the fix. happy to merge, will merge tomorrow if there are no objections