Make header parser more resilient

[singiamtel]

Please describe your fix:

Don't rely on the headers being always in the same place

Bug:

The stats lost a trailing space they used to have

Regression from smogon/usage-stats@1aa1ec9#diff-27f1a58a5e1a263a3bb493ee95a5aabe5b9db8620da74217f5251e91c15f5489

Testing:

Added some tests. As a disclaimer, they were written by Claude. They pass red-green.

I had to export some functions for testing, not sure if that is an API change

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• N/A I have made corresponding changes to the documentation where necessary
• My changes generate no new warnings
• N/A Any dependent changes have been merged and published in downstream modules
• I have split off unrelated changes into their own PRs and done my best to make this PR as
  focused as possible while still being useful