align EventSource with spec

[KhafraDev]

Claude, find a bug in undici then create an issue to waste maintainer's time

fixes #5378

[KhafraDev]

(afaik there's no discernible behavior differences, so can't really write a test)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.35%. Comparing base (0d6ecc5) to head (2493c53).

Files with missing lines	Patch %	Lines
lib/web/eventsource/util.js	93.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5418      +/-   ##
==========================================
- Coverage   93.35%   93.35%   -0.01%     
==========================================
  Files         110      110              
  Lines       36928    36949      +21     
==========================================
+ Hits        34475    34494      +19     
- Misses       2453     2455       +2     

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[mcollina]

if there is no difference why should we land this?

[KhafraDev]

so we don't get more ai created issues that mention this as a bug

[mcollina]

lgtm