Improve Chrome importer marker payload logic

[canova]

Fixes #5715.

This PR does three things:

1. Puts the category properly to the marker categories.
2. Removes the dummy payload if the event doesn't have any data.
3. Removes beginEventDetail all together with all the logic around it.

It's a relatively small PR, but test changes make it look bigger.

Example chrome profile:
Trace-20251212T152428.json.zip

Before / after

It looks like the chrome categories include commas, but this way still looked like an improvement to me.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.64%. Comparing base (450121b) to head (1b6b00e).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
src/profile-logic/import/chrome.ts	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5717      +/-   ##
==========================================
+ Coverage   85.63%   85.64%   +0.01%     
==========================================
  Files         319      319              
  Lines       31197    31197              
  Branches     8497     8588      +91     
==========================================
+ Hits        26714    26718       +4     
+ Misses       4053     4049       -4     
  Partials      430      430              

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[mstange]

I don't know what the beginEventDetail code did but I'll trust you that it's not needed. Oh, were we combining the JSON fields from the begin data end the end data? And now we're relying on the fact that this is just how we treat marker data anyway?